A Minecraft plugin is a JAR file (a Java archive) in the plugins/ folder and is always built for specific Minecraft versions, a specific Java version and usually also for specific server software such as Paper or Spigot. If any of these doesn't match, the plugin won't load at all, shuts itself off or individual features stop working. Each section below covers one specific message: what it looks like, what is behind it and how you fix it.

You can find the full message in the Console or under Files in logs/latest.log. Also read the lines before it, as they often name the affected plugin. We automatically set the Java version to match the Minecraft version. The plugin's download page lists which versions it supports. Before you change the version or edit configuration and world files, create a backup under Backups.

Plugin or Server Needs a Newer Java Version (UnsupportedClassVersionError)

What the message looks like:

[ModernPluginLoadingStrategy] Could not load plugin '<Plugin>.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: <Class> has been compiled by a more recent version of the Java Runtime (class file version 69.0), this version of the Java Runtime only recognizes class file versions up to 65.0

What is behind it: Java programs are compiled for a specific Java version. The file in the message requires a newer Java version than your server is currently using. The numbers tell you which: the number minus 44 gives the Java version, so 69 stands for Java 25 and 65 for Java 21. If the message names a plugin, that plugin won't start. If it affects the server JAR itself, the whole server won't start.

How to fix it:

  1. Read the file name after Could not load plugin. That is the file that doesn't match.
  2. On the plugin page, download the release built for your Minecraft version.
  3. Replace the old file under Files in the plugins/ folder and restart the server.
  4. If you need exactly this plugin release, switch the server under Version to the Minecraft version it requires. We automatically set the matching Java version per Minecraft version. Create a backup first.

Good to know: The plugins/.paper-remapped folder is a working copy made by Paper. Always replace the file directly in plugins/.

In detail: Which Java Version Does Your Minecraft Server Need?

Plugin Is Built for a Newer Minecraft Version (Unsupported API version)

What the message looks like:

[ModernPluginLoadingStrategy] Could not load plugin '<Plugin>.jar' in folder 'plugins/.paper-remapped'
org.bukkit.plugin.InvalidPluginException: Unsupported API version 26.1

What is behind it: Every plugin comes with a small profile file, the plugin.yml. Under api-version it states which server version the plugin was written for. If this version is newer than your server, Paper doesn't load the plugin at all, for example with Unsupported API version 26.1 on a 1.21.11 server. The server starts, but everything the plugin was supposed to provide is missing completely: commands, protection rules, menus.

How to fix it:

  1. Compare the number after Unsupported API version with your server's Minecraft version under Version.
  2. On Modrinth or SpigotMC, filter the plugin's downloads by your Minecraft version and download the matching release.
  3. Replace the old file under Files in the plugins/ folder and restart.
  4. If there is no matching release, switch the server under Version to the version the plugin requires. Create a backup first, because the world is converted to the new version in the process.

Good to know: Don't change the api-version in the plugin file yourself. The plugin will then load, but it uses features your server doesn't have and later fails with other errors.

More on this: Switching Minecraft Version and Mod Loader Safely

Older Paper Server Can't Convert a New Plugin (Fatal error trying to convert)

What the message looks like:

Fatal error trying to convert Geyser-Spigot <Version>:org/geysermc/geyser/platform/spigot/GeyserSpigotPlugin.class
java.lang.IllegalArgumentException: Unsupported class file major version 65

What is behind it: Paper version 1.19.4 or older converts plugins on loading with a built-in converter. This converter can't read classes compiled for Java 21 or newer (major version 65 stands for Java 21, 69 for Java 25). The plugin is therefore not loaded. It often hits Geyser: then no Bedrock players can join. The server itself starts and Java players can still join.

How to fix it:

  1. Read the name after trying to convert. That is the affected plugin.
  2. If you want to stay on your Minecraft version, download an older release of the plugin that was built for exactly this version and replace it under Files in the plugins/ folder.
  3. Alternatively, switch the server under Version to Minecraft 1.20.5 or newer. Newer Paper builds can read Java 21 classes. Create a backup first.
  4. Restart and check in the Console whether the plugin loads without errors.

Good to know: The message often appears several times in a row, once per class. Still, only that one plugin is affected.

In detail: Which Java Version Does Your Minecraft Server Need?

Plugin Reports: This Server Version Is Not Supported

What the message looks like:

[Essentials] You are running an unsupported server version!
[DecentHolograms] Plugin will now be disabled.
This plugin does not support Paper servers!

What is behind it: On startup, the plugin checks which Minecraft version and which server software it is running on. If neither is on its list, it warns or shuts itself off entirely or partially. The cause can be a version that is too new or too old, or the wrong server software. The server itself keeps running.

How to fix it:

  1. Enter plugins in the Console. Plugins shown in green are active, those shown in red are disabled.
  2. Check the plugin page for which Minecraft versions and which server software are supported.
  3. Download the matching release and replace the old file under Files in the plugins/ folder.
  4. If there is none, choose a Minecraft version the plugin supports under Version. If it requires different server software, you can do that via Change software / loader. Create a backup first.

Good to know: If the plugin stays green despite the warning, the basic features usually work. Test the commands you need and get the update as soon as it is released.

Paper Can't Adapt an Older Plugin to the Server Version (PluginRemapper)

What the message looks like:

[PluginRemapper] Encountered exception remapping plugins
java.util.concurrent.CompletionException: java.lang.RuntimeException: Failed to remap plugin jar 'plugins/<Plugin>.jar'

What is behind it: Newer Paper versions automatically rewrite older plugins on startup to the internal structure of the current Minecraft version. This rewriting is called remapping, and Paper stores the finished copies in plugins/.paper-remapped. For at least one plugin this has failed. The server keeps starting, but the plugin may not work at all or only partially, often without any further error message.

How to fix it:

  1. Read the file name after Failed to remap plugin jar.
  2. If the name contains something like (1), the file is probably in the folder twice. Remove the extra copy under Files.
  3. Download a release of the plugin that is explicitly built for your Minecraft version. Then Paper doesn't have to rewrite anything. Use it to replace the old file in plugins/.
  4. Restart and check whether the message is gone.

Good to know: If there is no matching release, the plugin isn't ready for your server version yet. Remove it or wait for an update.

Duplicate files: Plugin File Corrupted, Duplicated or Not a Plugin at All

Unknown Block or Item Name in a Plugin Configuration (Unsupported key)

What the message looks like:

Unsupported key: minecraft:bed

What is behind it: Minecraft lists every block and every item under a fixed name of the form minecraft:<name>, the so-called registry key. A plugin or its configuration uses a name that your Minecraft version doesn't know. A typical example is minecraft:bed: since 1.13, beds are named after their color, such as red_bed. The entry is skipped, and the server and the rest of the plugin keep running.

How to fix it:

  1. Look at the lines before the message in the Console or in logs/latest.log. They show which plugin is currently loading.
  2. Under Files, open this plugin's configuration in the plugins/<Plugin>/ folder and search for the named entry.
  3. Replace it with the current name, which you can find in the Minecraft Wiki. Configurations in YAML format (.yml) are sensitive: leave indentation and quotation marks unchanged.
  4. Restart the server. If the name isn't in your configuration but in the plugin itself, only a plugin update helps.

For errors in the file: A Plugin's Configuration File Is Faulty

Plugin Needs a Newer Text Library (Adventure) Than Your Server

What the message looks like:

Caused by: java.lang.ExceptionInInitializerError: Exception java.lang.TypeNotPresentException: Type net.kyori.adventure.text.TranslationArgument not present

What is behind it: Adventure is the library Paper uses to create colored chat text, titles and hover texts. It is built into the server, so its version depends on your Minecraft version. The plugin was built against a newer Adventure version, and the named class is missing on your server. Parts of the plugin therefore don't start. The server keeps running.

How to fix it:

  1. Find out which plugin is affected. The stack trace, meaning the list of at … lines below the message, often contains the JAR file name. Otherwise the lines before it help.
  2. Download an older release of the plugin that is built for your server version and replace the file under Files in the plugins/ folder.
  3. Or switch the server under Version to a newer Minecraft version. Create a backup first.

Good to know: An additional Adventure file in the plugins/ folder doesn't help, because Paper uses its built-in version.

ProtocolLib Doesn't Match the Server Version

What the message looks like:

[ProtocolLib] INTERNAL ERROR: Cannot load ProtocolLib.
  If this problem hasn't already been reported, please open a ticket
  java.lang.IllegalArgumentException: Unable to find a field that matches …

What is behind it: ProtocolLib is a helper library that other plugins use to read and modify network packets. To do this, it reaches deep into the server's internals, and these change with almost every Minecraft version. If ProtocolLib doesn't find the expected packet fields, it doesn't load. All plugins that need ProtocolLib then don't work either.

How to fix it:

  1. Download the latest ProtocolLib build for your current Minecraft version. For very new versions, matching builds are often available first as a pre-release on the project page.
  2. Replace the old file under Files in the plugins/ folder. Keep only one ProtocolLib file.
  3. Restart the server completely, meaning stop and start it again, not with /reload.
  4. Enter plugins in the Console and check whether ProtocolLib and the dependent plugins are green.

Good to know: The request to open a ticket with ProtocolLib comes from the plugin itself. Usually the matching build is simply missing.

Dependent plugins: Plugin Needs Another Plugin That Is Missing

Citizens Shuts Itself Off: Build Doesn't Match the Minecraft Version

What the message looks like:

[Citizens] v<Version> (build <Number>) is not compatible with Minecraft <Version> - try upgrading or downgrading Citizens. Disabling.

What is behind it: Citizens is a plugin for NPCs, meaning computer-controlled characters such as merchants or quest givers. Each Citizens build only supports certain Minecraft versions and server builds. If it doesn't match your server, Citizens disables itself on startup. Your NPCs then don't appear, and plugins built on Citizens lose that functionality. The server itself keeps running.

How to fix it:

  1. Check under Version which Minecraft version your server has.
  2. Download the Citizens build for exactly this version. Many Citizens builds have SNAPSHOT in their name; what matters is the Minecraft version the download page lists for it.
  3. Replace the old file under Files in the plugins/ folder and restart.
  4. If there is no matching build, adjust the server version under Version. Create a backup first.

Good to know: try upgrading or downgrading means: Citizens can be too old or too new. So the newest file isn't automatically the right one.

Plugin Reports: Not Compatible With Your Minecraft Version

What the message looks like:

[Citizens] v<Version> (build <Number>) is not compatible with your version of Minecraft. Disabling...
[<Plugin>] Plugin will be disabled!
Your MC version isn't supported

What is behind it: The plugin is built for a different Minecraft version or server software and reports this itself. Similar wordings are This plugin only supports Spigot-API … or Unable to load Adventure, chat components will not work. Depending on the plugin, it either shuts down completely or individual features stop working, such as colored chat text. The server keeps running.

How to fix it:

  1. Read the plugin name in the square brackets at the start of the line. If it's missing, it appears in the lines before.
  2. Check the plugin page for which Minecraft versions and which server software it supports.
  3. Download the matching release and replace the old file under Files in the plugins/ folder.
  4. If there is none, switch the server under Version to the version the plugin requires. Create a backup first.
  5. After the restart, enter plugins in the Console and check whether the plugin is green.

Good to know: Switching to a newer Minecraft version converts the world. Going back is only reliable via the backup.

Plugin Needs a Newer Paper Build of Your Minecraft Version

What the message looks like:

[EntrypointUtil] Class io.papermc.paper.plugin.loader.library.impl.MavenLibraryResolver does not have member field 'java.lang.String MAVEN_CENTRAL_DEFAULT_MIRROR'
java.lang.NoSuchFieldError: …

What is behind it: Paper is continuously developed even within a Minecraft version, and each release is called a build. The plugin accesses a field of the Paper API, specifically in the part that downloads additional libraries for plugins at startup. This field (MAVEN_CENTRAL_DEFAULT_MIRROR) only exists in newer builds. It is missing in your build, so the plugin doesn't load. The server itself keeps running.

How to fix it:

  1. In the panel, open Version and update to the latest Paper build of your Minecraft version.
  2. Restart and check in the Console whether the plugin now loads without errors.
  3. If you don't want to change the build, use an older release of the plugin and replace the file under Files in the plugins/ folder.

Good to know: A newer build of the same Minecraft version is a small step. The world stays on the same Minecraft version. A backup beforehand doesn't hurt anyway.

Plugin Can't Find Internal Server Parts (Failed to init reflection)

What the message looks like:

Failed to init reflection: Index 3 out of bounds for length 3

What is behind it: Some plugins don't only use the server's official interface, but read internal classes directly by their name. This is called reflection. These internals change with almost every Minecraft version. If the plugin doesn't find what it expects, it was built for a different server version, and the features that depend on it stop working. The server keeps running.

How to fix it:

  1. The message itself doesn't name a plugin. Look at the lines directly before it in the Console or in logs/latest.log. They show which plugin was just loaded.
  2. Update this plugin to a release for your server version and replace the file under Files in the plugins/ folder.
  3. Restart and check whether the message is gone.

Good to know: If there is no newer release, the plugin is probably no longer maintained. In that case, a current replacement plugin with the same function is worth it.

A Plugin's Glow Effects Are Disabled (GlowingEntities)

What the message looks like:

[GlowingEntities] [GlowingEntities] Glowing Entities reflection failed to initialize. The util is disabled. Please ensure your version (<Version>) is supported.
java.lang.NoSuchFieldException: MARKER

What is behind it: GlowingEntities isn't a standalone plugin, but a building block that other plugins bundle to make players, mobs or objects glow. The building block can't find an internal field of your server build. That's why the glow effects of the affected plugin are disabled. The server keeps running, and usually the plugin keeps working without the effects.

How to fix it:

  1. Find the plugin that bundles GlowingEntities. In the at … lines below the message there is an entry of the form <Plugin>.jar//….
  2. Download this plugin in a release for your Paper build.
  3. Replace the old file under Files in the plugins/ folder and restart.

Good to know: You can't update GlowingEntities separately. The developer of the plugin that contains the building block takes care of that. If you don't need the glow effects, you can ignore the message until the next update.

Simple_RTP: Heads in the Menu Aren't Displayed

What the message looks like:

[Simple_RTP] Failed to apply GUI head texture: IllegalArgumentException: Can not set net.minecraft.world.item.component.ResolvableProfile field org.bukkit.craftbukkit.inventory.CraftMetaSkull.profile to com.mojang.authlib.GameProfile

What is behind it: Simple_RTP is a plugin for random teleports. For the heads in its menu, it sets a player profile in the old format (GameProfile). Your Minecraft version, however, expects a newer format (ResolvableProfile) at this point. The two don't match, so the heads in the menu don't work. The server keeps running.

How to fix it:

  1. Check under Version which Minecraft version your server has.
  2. Download Simple_RTP in the release for this Minecraft version.
  3. Replace the old file under Files in the plugins/ folder and restart.
  4. Open the menu in the game and check whether the heads now appear.

Good to know: The message may repeat every time the menu is opened. It causes no further harm; it only shows that the plugin release is outdated.

ZNPCs: Saved NPCs Don't Match the Plugin Version

What the message looks like:

io.github.gonalez.znpcs.commands.CommandExecuteException
No enum constant io.github.gonalez.znpcs.npc.<…>.PLAYER

What is behind it: ZNPCs saves your NPCs, including type and actions, in a file. It contains a type or an action that your installed ZNPCs version doesn't know, for example ActionType.PLAYER. This usually happens when the NPCs were created with a different ZNPCs version. The executed command aborts, and the server keeps running.

How to fix it:

  1. Create a backup under Backups.
  2. Install the ZNPCs version the NPCs were created with and restart.
  3. If that's not possible, stop the server, open the file plugins/ZNPCs/data.json under Files and remove the affected NPC entry. JSON is a text format with brackets and commas: a missing comma makes the whole file unreadable.
  4. Start the server and test the command again.

Good to know: Only edit the file while the server is stopped. If it is running, ZNPCs can overwrite your change again when it shuts down.

Plugin Needs Paper or a Newer Version (failed to register events)

What the message looks like:

Plugin <Plugin> v<Version> has failed to register events for class <Class> because io/papermc/paper/event/<Event> does not exist.

What is behind it: Plugins react to events in the game, for example when a player breaks a block. This plugin wants to listen for an event that your server software doesn't know. Usually the plugin is written for Paper, but the server runs on Spigot. Or the event only exists in a newer Minecraft version. The affected part of the plugin doesn't work, and the server keeps running.

How to fix it:

  1. Read the name after because. If it starts with io/papermc/paper or com/destroystokyo/paper, the plugin needs Paper.
  2. If your server runs on Spigot, switch it to Paper under Change software / loader. Plugins for Spigot generally also run on Paper. Create a backup first.
  3. If it already runs on Paper, the event is newer than your version. Use a release of the plugin for your Minecraft version or switch the server under Version.
  4. Restart and check whether the message is gone.

More on this: Switching Server Software and Version in the Panel

File Is Built for a Newer Java Version (Unsupported class file major version)

What the message looks like:

java.lang.IllegalArgumentException: Unsupported class file major version 69

What is behind it: Plugins, mods and libraries are compiled for a specific Java version. The number after major version tells you which: the number minus 44 gives the Java version, so 65 stands for Java 21 and 69 for Java 25. The tools of your server that read or rewrite plugins and mods on loading don't understand this newer version. The affected part isn't loaded.

How to fix it:

  1. In the lines before it, find out which plugin or mod was just being loaded.
  2. Download a release for it that is built for your Minecraft version and replace the old file under Files in the plugins/ or mods/ folder.
  3. Alternatively, update the server under Version to a newer Minecraft version that runs on Java 21 or newer. We set the matching Java version automatically. Create a backup first.

Good to know: If Fatal error trying to convert appears directly before it, the section on older Paper servers further up applies.

In detail: Which Java Version Does Your Minecraft Server Need?

Old Plugin Looks for a Removed Chunk Function (Legacy IDs)

What the message looks like:

java.lang.NoSuchMethodException: net.minecraft.world.level.chunk.Chunk.k(net.minecraft.core.BlockPosition)

What is behind it: Up to Minecraft 1.12, blocks had numeric IDs; stone, for example, was 1. Some old plugins still work internally with these legacy IDs and access internal functions of the game directly for that, the classes under net.minecraft. The chunk function it is looking for no longer exists in your version. The plugin's legacy ID function therefore doesn't start. Depending on the plugin, this causes individual or many features to fail.

How to fix it:

  1. Look at the stack trace below the message, meaning the at … lines. It contains the name of the plugin or its JAR file.
  2. Update this plugin to a release for your Minecraft version and replace the file under Files in the plugins/ folder.
  3. If there is none, the plugin is no longer maintained for modern versions. Look for a current replacement plugin.

Good to know: The short name k is an obfuscated internal name. Such names change with every Minecraft version. Plugins that use them directly therefore break with almost every update.

Plugin Uses an Outdated Mojang Profile Function (Player Heads)

What the message looks like:

java.lang.NoSuchMethodException: com.mojang.authlib.GameProfile.getProperties()

What is behind it: authlib is Mojang's library that the server uses to manage player profiles with name and skin. The plugin reads these profiles through a method (GameProfile.getProperties) that no longer exists in your Minecraft version. The affected feature fails, usually the player heads in menus. The server keeps running.

How to fix it:

  1. Find the plugin: its name or the name of its JAR file appears in the at … lines below the message or in the lines before it.
  2. Download a release of this plugin for your Minecraft version.
  3. Replace the old file under Files in the plugins/ folder and restart.
  4. Open the affected menu in the game and check whether the heads appear again.

Good to know: This change hits many menu and head plugins at the same time. If several plugins no longer show heads, it is usually the same cause, and each one needs its own update.

Plugin Looks for Blocks, Particles or Attributes That Don't Exist in Your Version

What the message looks like:

java.lang.IllegalArgumentException: No enum constant org.bukkit.Material.<NAME>
Unknown material '<NAME>' for button
java.lang.NoSuchFieldError: Class org.bukkit.<Class> does not have member field '<Field>'

What is behind it: Every Minecraft version brings new blocks, particles and attributes and renames some. The plugin is built for a different version and accesses something that doesn't exist here or only under a different name. Depending on the plugin, the error appears with every action, or individual features fail. Could not initialize Brigadier mappings also belongs here. Brigadier is Minecraft's command system, which provides tab completion, among other things.

How to fix it:

  1. Read the plugin name in the lines before it or in the at … lines below.
  2. Download the plugin release that matches your Minecraft version and replace the file under Files in the plugins/ folder.
  3. Or switch the server under Version to the Minecraft version the plugin requires. Create a backup first.
  4. If the unknown name is in a configuration file of the plugin, for example for menu buttons, you can replace it there with the current name.

Marriage Plugin Doesn't Match the Minecraft Version

What the message looks like:

[Marriage] Marriage is not compatible with the version of minecraft you are using
Marriage kissing will be disabled!

What is behind it: Marriage lets players marry in the game and brings features such as kissing. The plugin doesn't support your Minecraft version. It therefore disables at least the kissing feature, and other parts may be affected as well. The server keeps running normally.

How to fix it:

  1. Check the plugin page for whether there is a Marriage release for your Minecraft version. You can see your version under Version.
  2. If there is one, replace the old file under Files in the plugins/ folder and restart.
  3. If there is none, replace the plugin with a current one with a similar function. Expect that another plugin won't take over Marriage's saved data, and create a backup first.

Good to know: If only the missing kissing feature bothers you and the rest works, you can ignore the message until an update is released.

Anticheat or Packet Plugin Doesn't Fully Know Your Minecraft Version Yet

What the message looks like:

java.lang.IllegalStateException: Unknown entity metadata type id: <Number> version V_<Version>

What is behind it: Some plugins read the network packets between the game and the server, for example the anticheat GrimAC via the PacketEvents library. New Minecraft versions add new data types for creatures and objects, the entity metadata. The plugin doesn't know one of them yet, usually because it is older than your server version. Affected packets are processed incorrectly. This can cause false alarms from the anticheat or connection problems.

How to fix it:

  1. Update the plugin and, if it is installed as a separate file, also PacketEvents to a version that explicitly supports your Minecraft version.
  2. Restart the server completely.
  3. If there is no matching version yet, disable the plugin temporarily: rename its file under Files, for example to <Plugin>.jar.off. The server only loads files ending in .jar.
  4. Alternatively, use a Minecraft version under Version that the plugin supports. Create a backup first.

WorldEdit Add-On Plugin Uses a Removed Interface

What the message looks like:

[WorldEdit] Got request to set EditSessionFactory <…> EditSessionFactories have been removed …

What is behind it: WorldEdit removed an older interface, the EditSessionFactory. Through it, add-on plugins could hook into every WorldEdit change. An add-on plugin, for example for block logs or area protection, still tries to use it. WorldEdit itself works normally. However, the add-on plugin's logging or protection feature doesn't apply to WorldEdit changes: whatever you build or delete with WorldEdit is neither recorded nor blocked by it.

How to fix it:

  1. Find out which plugin makes the request. The name is usually in the message itself or in the lines before it.
  2. Update this add-on plugin to a version that works with your WorldEdit or FastAsyncWorldEdit (FAWE, a faster WorldEdit variant).
  3. If there is none, turn off the WorldEdit integration in the add-on plugin's configuration, if there is a setting for it.
  4. Restart and check whether the message is gone.

Good to know: Until this is fixed, you can't rely on WorldEdit changes being logged or being able to be rolled back through it.

Braslety: Recipe Isn't Registered

What the message looks like:

[Braslety] … java.lang.IllegalAccessException: class com.example.braslety.BraceletsPlugin cannot access …

What is behind it: Braslety accesses internal CraftBukkit classes via reflection. Reflection means: the plugin reads parts of the server directly instead of using the official interface. In your server version, these parts aren't accessible, and Java denies the access. The affected recipe is therefore not registered, so the corresponding item can't be crafted. The server keeps running.

How to fix it:

  1. Check whether there is a Braslety version for your current Paper build. You can see your version under Version.
  2. If there is one, replace the old file under Files in the plugins/ folder and restart.
  3. If there is none, replace the plugin with a maintained one with a similar function.
  4. Check in the game whether the recipe works.

Good to know: The package name com.example points to a self-made or little-used plugin. In that case only its creator can deliver an update.

ViaVersion Can't Load a Translation Module

What the message looks like:

[ViaVersion] Error during loading of Protocol<Version>To<Version>

What is behind it: ViaVersion translates between game versions so that players with newer or older clients can join too. Extensions such as ViaBackwards add this for older clients. Each translation lives in its own module whose name begins with Protocol. One of them couldn't be loaded, usually because ViaVersion and its extensions don't match each other or the server version. Players with the affected client versions then can't join. Players on exactly the server version aren't affected.

How to fix it:

  1. Download the latest versions of ViaVersion and of all Via extensions you use.
  2. Replace them all together under Files in the plugins/ folder. Don't mix old and new Via files.
  3. Restart the server completely.
  4. Test joining with the client version that didn't work before.

In detail: Setting Up ViaVersion: Connecting Older and Newer Clients

LibreLogin Doesn't Run on Spigot

What the message looks like:

[LibreLogin] Detected an unsupported server distribution. Please use Paper or its forks. SPIGOT IS NOT SUPPORTED!

What is behind it: LibreLogin is a login plugin: players sign in with a password. This is especially important on servers in offline mode. LibreLogin needs Paper or a compatible fork, meaning a variant of Paper such as Purpur, and shuts itself off on Spigot. If your server runs in offline mode, password protection is missing as a result: players can connect under other people's names.

How to fix it:

  1. Create a backup under Backups.
  2. Switch the server under Change software / loader to Paper or Purpur. Plugins for Spigot generally keep running there.
  3. Restart and check in the Console whether LibreLogin loads without this message.
  4. If you want to stay on Spigot, use a login plugin that explicitly supports Spigot.

Good to know: If your server runs in offline mode, fix this before players join.

More on this: Switching Server Software and Version in the Panel

Nova Is Missing a Java Startup Parameter (-javaagent)

What the message looks like:

[Nova] Missing required JVM argument: -javaagent:plugins/Nova-<Version>.jar

What's behind it: Nova is an extensive plugin that makes custom blocks and items possible on Paper servers. For some of its features, Java has to load a so-called Java agent at startup. This is an add-on that becomes active before the actual server and is included via a startup parameter. Your server starts without this parameter. Nova therefore doesn't work completely, while the server itself runs.

How to fix it:

  1. Read the exact parameter from the message, including the file name of the Nova JAR.
  2. You cannot change the startup command of your Minecraft server in the panel yourself. Open a ticket, paste the parameter from the message and include the time of the message.
  3. If you don't strictly need Nova, remove the plugin under Plugins and restart the server. The message then disappears.

Good to know: With every new Nova version, the file name in the parameter usually changes as well. After a Nova update, the message therefore often returns.

Your message isn't listed?

Read the whole message in the Console or in logs/latest.log, including the lines before it. You can recognize the affected plugin by the name in square brackets at the start of the line or in the stack trace, the list of at … lines: it contains JAR names from plugins/ or mods/ and package names that often include the plugin name. Then compare the supported versions on the plugin page with your server. Create a backup before every version change. If you get stuck, open a ticket and include the time of the message.