Forge, NeoForge, and Fabric are mod loaders: on startup they load all mods from the mods/ folder and check whether they fit together and match the Minecraft version. If something goes wrong, a message ends up in the server log. Each section below covers exactly one of these messages. Errors that prevent startup come first, followed by warnings where the server keeps running.

You can find the full message in the Console or under Files in logs/latest.log, and crash reports in the crash-reports/ folder. Many messages are followed by a stack trace: a list of program locations showing where the error occurred. Package and folder names in it often reveal the mod involved. Before touching world or configuration files, create a backup under Backups.

Incompatible mods prevent startup ("Incompatible mods found")

What the message looks like:

Incompatible mods found!
net.fabricmc.loader.impl.FormattedException: Some of your mods are incompatible with the game or each other!
     - Install <Mod>, version <Version> or later.

What's behind it: Before startup, the mod loader checks all mods and has found missing or mismatched dependencies. A dependency is another mod that a mod needs in order to run, often a library. The server won't start like this. Right below the message, it says what is missing or causing trouble.

How to fix it:

  1. In the Console or in logs/latest.log, read the lines below A potential solution has been determined.
  2. Install <Mod>, version <Version> or later means: this mod is missing or too old. Install it under Mods in at least this version, matching your Minecraft version and loader.
  3. Replace 'Minecraft' (minecraft) <Version> with any <Version> version means: a mod is built for a different Minecraft version. Get its build for your version or remove it. Only change the server version under Version if all mods match it, and only after creating a backup.
  4. If a line mentions 'OpenJDK 64-Bit Server VM' (java), a mod requires a different Java version. Here too, the mod's build for your Minecraft version usually helps.

Good to know: Work through the whole list and only then restart; this saves startup attempts. In detail: Setting up Minecraft mods and modpacks correctly.

NeoForge aborts startup ("Couldn't find Minecraft server thread")

What the message looks like:

net.neoforged.fml.startup.FatalStartupException: Couldn't find Minecraft server thread. Startup likely failed.
    at net.neoforged.fml.startup.Server.main(Server.java:23)

What's behind it: This message is a consequence, not the cause. An earlier error from a mod or the loader ended startup before NeoForge could create the actual Minecraft server. That's why the server doesn't start. The line itself doesn't name a mod; the trail lies further up in the log.

How to fix it:

  1. Under Files, open the file logs/latest.log. If there's a crash report, also look at the newest file in crash-reports/.
  2. Starting from this message, go upward to the first line containing Exception, ERROR, or Caused by. The first one is the decisive one; later errors are usually consequences of it.
  3. Look for the mod name there. It appears in the message itself, as a file name ending in .jar, or as a package name in the stack trace below.
  4. Update or remove this mod under Mods. If it requires a different NeoForge version, set a matching one under Version.
  5. Restart the server and check whether the message has disappeared.

Good to know: If you find a message from another section of this article further up, fix that one first. The line here will then disappear on its own.

Fabric mod crashes on startup ("Could not execute entrypoint stage")

What the message looks like:

Failed to start the minecraft server
java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by '<mod-id>' at '<Class>'!

What's behind it: An entrypoint is the point where Fabric starts a mod's program. The mod behind provided by already crashed in this early phase, so the server doesn't fully start up. The exact reason is given by the Caused by: lines below, often a missing dependency or a mismatched version.

How to fix it:

  1. In the Console or in logs/latest.log, read the name behind provided by, for example geyser-fabric.
  2. Look at the Caused by: lines below. They show whether something is missing or doesn't fit.
  3. Update the mod and its dependencies under Mods to match your Minecraft and Fabric version. Many Fabric mods additionally need the Fabric API.
  4. If the error appeared right after an installation, remove the mod as a test and restart. If the server then starts, this mod is the culprit.

Good to know: How Fabric Loader, Fabric API, and mods fit together is explained in Fabric Loader and Fabric API: Resolving version errors.

Fabric mod has an invalid mixin configuration

What the message looks like:

Error parsing or using Mixin config <mod-id>.mixins.json for mod <mod-id>

What's behind it: Mixin is a technique mods use to modify specific places in the Minecraft program at startup. Which places these are is defined in a mixin configuration inside the mod file. Fabric can't read or apply this configuration of the named mod, and server startup aborts. Usually the mod file doesn't match your Minecraft or Fabric Loader version exactly.

How to fix it:

  1. In the Console or in logs/latest.log, read the mod name behind for mod.
  2. Check under Version which Minecraft version your server uses.
  3. Replace the mod under Mods with the build offered for exactly this Minecraft version and for Fabric. Make sure the old file disappears from mods/.
  4. Restart the server.

Good to know: Even small version jumps count: a mod for a different minor version of Minecraft can trigger exactly this error even though the major version is the same.

Forge aborts loading mods (LoaderException)

What the message looks like:

net.minecraftforge.fml.common.LoaderException: <Reason>

What's behind it: Forge couldn't load a mod and aborted the startup process. Typical triggers are a mod that doesn't match the Minecraft or Forge version, a missing dependency, or an invalid value in a mod configuration. The server won't start in this state. This form of the message mostly occurs with older Forge versions, as used by many classic modpacks.

How to fix it:

  1. In the Console or in logs/latest.log, read the lines directly below the message. They usually contain the mod name and the specific reason.
  2. If you just added a mod, remove it under Mods or replace it with the version for your Minecraft and Forge version.
  3. If the error appeared right after a change to a file in the config/ folder, undo that change.
  4. Restart the server.

Good to know: If the lines mention a missing mod, Plugin or mod needs a missing extension can also help.

Server won't start: error while loading mods

What the message looks like:

Failed to start the minecraft server
Missing or unsupported mandatory dependencies:
    Mod ID: '<mod-id>', Requested by: '<Mod>', Expected range: '[<Version>,)', Actual version: '[MISSING]'

What's behind it: The server aborts startup because a mod is missing, present in the wrong version, or crashes while loading. As long as the error persists, the server is unreachable. The lines below the message name the mod and the required version.

How to fix it:

  1. In the Console or in logs/latest.log, read the lines below the message. Mod ID is the required mod, Requested by is the mod that requires it, and Expected range is the required version.
  2. Actual version: '[MISSING]' means: not installed. If a version number appears there, the existing build is too old or too new.
  3. Install the dependency under Mods in the required version, or remove the mod that requires it.
  4. If the line refers to forge, neoforge, or minecraft, the loader or game version doesn't match. Set a matching one under Version or Change software / loader.
  5. Restart the server.

Good to know: [<Version>,) means "this version or newer". If only Exception in thread "main" java.lang.reflect.InvocationTargetException appears, look below it for Caused by:. More on this: Changing server software and version in the panel.

Forge: mod or configuration file (.toml) is unreadable

What the message looks like:

com.electronwill.nightconfig.core.io.ParsingException: Failed to parse data from Reader

What's behind it: TOML is a simple text format for settings (name = value). On startup, Forge reads the description of each mod (mods.toml in the mod file) and all .toml configuration files in UTF-8 character encoding. If a file is encoded differently, for example with umlauts or Chinese characters from a Windows editor, or was damaged during upload, Forge aborts before the world loads. The server then shuts down again a few seconds after starting. Often the most recently added mod is to blame, especially one intended only for the game client.

How to fix it:

  1. Create a backup under Backups.
  2. Under Mods, remove the most recently added mod and restart. If the server starts, it was the cause. Then install a server version of this mod or a different version.
  3. If it still doesn't start, under Files open the most recently edited .toml file in the config/ folder. Save it again without special characters (UTF-8) or delete it. Forge recreates it with default values on the next start; your changes in it will then be gone.
  4. If you get stuck, open a ticket and provide the time of the message and the name of the most recently installed mod.

Good to know: How to open and save files in the panel is shown in Editing server files in the panel and connecting via SFTP.

Recipe could not be loaded ("Parsing error loading recipe")

What the message looks like:

Parsing error loading recipe <mod>:<rezept>: com.google.gson.JsonSyntaxException: Unknown item '<other-mod>:<item>'

What's behind it: A recipe refers to an item or recipe type that doesn't exist in the current modpack. This often happens after a mod update or when an add-on that the recipe requires is missing. The server continues to start normally; only this one recipe is missing in the game. In large modpacks such lines are common, because many mods ship recipes for partner mods that aren't installed at all.

How to fix it:

  1. Read the line from left to right: after loading recipe, the part before the colon is the mod that provides the recipe. After Unknown item, the missing item appears, and the part before the colon is the mod it belongs to.
  2. If you need the item or recipe in the game, check under Mods whether the corresponding mod is missing or outdated.
  3. Install or update it to match your Minecraft and loader version, and restart.
  4. If you don't need the recipe, you can ignore the message.

Good to know: If the reason is No key id in …, the recipe is written for an older recipe format. A build of the mod for your Minecraft version usually helps.

Cobblemon: player data corrupted

What the message looks like:

.old File cobblemonplayerdata for [<UUID>] is corrupt or missing. Data is lost
.old File pokedex for [<UUID>] is corrupt or missing. Data is lost

What's behind it: Cobblemon stores separate files for each player, for example for the Pokédex, party, and spawn data. One of them is damaged, usually after a crash while saving, and the backup copy with the .old extension is also unusable or missing. This player's Cobblemon data is lost; Cobblemon creates it anew. The server itself keeps running.

How to fix it:

  1. Find out who is affected: the file usercache.json in the main folder maps the UUID from the message to a player name.
  2. If the data is important, stop the server and restore a backup from before the incident under Backups. The data is located in world/pokemon or world/cobblemonplayerdata (world is the default name of the world folder).
  3. Keep in mind: a full backup resets the entire world to its state at that time, including everything other players have built since.
  4. In future, create backups regularly, especially before updates.

Good to know: If such messages pile up, the server is probably crashing more often. Then look for the cause under Server health and in crash-reports/. Guide: Creating and restoring a server backup.

A mod's data file contains faulty JSON

What the message looks like:

Couldn't parse data file <mod>:<name> from <mod>:recipes/<name>.json
com.google.gson.JsonParseException: com.google.gson.stream.MalformedJsonException: Unterminated object at line 9 column 40

What's behind it: JSON is a text format for data using curly braces, quotation marks, and commas. A mod or datapack supplies a recipe, data, or configuration file in which a character is missing or superfluous. Minecraft skips this content; the rest of the server keeps running.

How to fix it:

  1. Read the first line: the part before the colon after from names the mod or datapack, followed by the file path.
  2. If the file comes from a mod, update that mod under Mods.
  3. If you edited the file yourself, for example in your own datapack under world/datapacks/, open it under Files. line and column give the line and column of the error.
  4. A missing comma or an unclosed bracket is typical. If you can't find the error, restore the original file.
  5. Restart the server.

Good to know: Unterminated object means that a comma or a closing curly brace was expected at this point. More on datapacks: Datapack and resource pack errors.

Forge mod contains a faulty language file

What the message looks like:

Skipped language file: <mod-id>:lang/en_us.json
com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 4 column 3 path $

What's behind it: Language files contain a mod's texts, such as item names. Forge couldn't read one of these JSON files in the mod file (.jar) and skipped it. In the stack trace below, LanguageHook appears, the part of Forge that loads language files. The server keeps running; only the texts from this file are missing on the server.

How to fix it:

  1. Read the mod ID before the colon after Skipped language file:. If this line is missing, take the first mod mentioned around the message.
  2. Update this mod under Mods or replace it with a build for the same Minecraft and Forge version. A build is a finished, compiled version of the mod.
  3. Restart the server and check whether the line has disappeared.

Good to know: The hint Use JsonReader.setLenient(true) is aimed at mod developers; you can't configure anything with it. The file is inside the mod file, so repairing it yourself isn't worth it.

A mod's mixin failed ("Mixin apply for mod … failed")

What the message looks like:

Mixin apply for mod <mod-id> failed <mod-id>.mixins.json:<Class> from mod <mod-id> -> <Target class>:
org.spongepowered.asm.mixin.injection.throwables.InvalidInjectionException Critical injection failure: @Inject annotation on <Method> could not find any targets

What's behind it: Through mixins, mods modify certain places in the Minecraft program at startup. The named mod is looking for such a place but can't find it. Usually the mod doesn't match the Minecraft version, or another mod has already modified that place. The affected function of the mod may then not work properly.

How to fix it:

  1. Read the name behind from mod. That is the mod whose modification is failing.
  2. Check on the mod's page whether there is a build for your modpack or Minecraft version, and update it under Mods.
  3. If you recently added another mod that modifies similar areas, such as a performance mod, remove it as a test and restart.
  4. If the error persists, remove the mod or report it to the author with the full line from logs/latest.log.

Good to know: Entries like class_1799 or m_77607_ are internal names of Minecraft classes and methods. Only the mod name matters to you.

Mixin can't be applied: mods don't get along

What the message looks like:

org.spongepowered.asm.mixin.injection.throwables.InvalidInjectionException: @At("INVOKE") on <Target> with priority 1000 cannot inject into <Target>
Mixin apply failed <mod-id>.mixins.json:<Class> -> <Target class>

What's behind it: A mod wants to modify a place via mixin, that is, a targeted intervention in the Minecraft program. In this mod or Minecraft version, however, it doesn't find the expected place, often because a second mod has already changed something right there. The mods involved are not compatible with each other in these versions.

How to fix it:

  1. Collect the mods involved: the name of the mixin file (<mod-id>.mixins.json) reveals the first one. If the line contains merged by, the mixin file of the second one follows.
  2. Create a backup and update the named mods together under Mods, matching your Minecraft and loader version.
  3. If that doesn't help, roll one of the two back to a version that works with the other. The mods' pages often list known incompatibilities.
  4. Restart the server and check in the Console whether the message has disappeared.

Forge modlauncher conflicts on startup (ConcurrentModificationException)

What the message looks like:

Exception in thread "Thread-0" java.util.ConcurrentModificationException
    at MC-BOOTSTRAP/cpw.mods.modlauncher@<Version>/cpw.mods.modlauncher.LaunchPluginHandler…

What's behind it: The modlauncher is the part of Forge that loads the mods and rewrites Minecraft classes before startup. Several mod transformers, that is, building blocks for such rewrites, modified the same internal list at the same time, and Java aborts this. This points to incompatible coremods, meaning mods that intervene deeply in the loader, or to a bug in the loader itself.

How to fix it:

  1. Create a backup under Backups.
  2. Update Forge and all coremods. With a ready-made modpack, first check under Modpack whether a newer version exists instead of changing Forge individually.
  3. As a test, remove the most recently added performance or mixin mods under Mods and restart.
  4. Once the server starts cleanly again, add the mods back one by one until you know the trigger.

Good to know: Because this involves simultaneous access, the error doesn't occur on every start. If the server starts on the second attempt, the cause is still not fixed.

A mod's schema file has invalid metadata

What the message looks like:

Couldn't deserialize schema file at <mod-id>:schemas/<name>.json because the json file has invalid schema metadata.

What's behind it: Some mods describe content in their own JSON files, so-called schema files. Such a file from the named mod doesn't match the format the mod or its library expects, and is skipped. The server keeps running; only the content of this file is missing. Often the mod and a library mod don't match each other.

How to fix it:

  1. Read the mod ID before the colon after schema file at.
  2. Check the stack trace below to see which other mod is involved. Often this is a library the mod needs.
  3. Bring both to matching versions under Mods. The mod's page states which version of the library it expects.
  4. If the faulty file is in a datapack or a folder you created, you can remove it instead. Create a backup first.
  5. Restart the server.

YUNG's Better End Island: missing world state

What the message looks like:

key missing: bei_ExtraDragonFight in {BorderCenterX:0.0d,BorderCenterZ:0.0d,…}

What's behind it: The mod YUNG's Better End Island creates its own entry for the dragon fight in the world data. It currently can't find this entry. It then uses a default state and rewrites the entry on the next save. The rest of the world data is preserved, and the server keeps running.

How to fix it:

  1. Check in the game whether the End island and dragon fight work normally. If so, a clean restart is enough: stop the server normally and start it again, without killing it forcefully.
  2. If the fight state is wrong, update the mod and YUNG's API under Mods to matching versions.
  3. Create a backup of the world under Backups.
  4. Only then use the official command /end_island reset. Read on the mod's page beforehand what it resets.

Good to know: If the message appears once and not again afterward, the mod has rewritten the entry. See also Creating and restoring a server backup.

A mod's block properties are unreadable

What the message looks like:

Error while loading physics block properties entry: DataResult.Error['Not a JSON object: [{"selector":"<mod>:<block>","properties":{…}}]

What's behind it: A mod wanted to read properties for certain blocks, here for its physics, but the entry doesn't have the expected form. Not a JSON object followed by a square bracket means: the mod expects an entry in curly braces but finds a list. The entry usually comes from a configuration written for a different mod version. The affected blocks then behave like ordinary blocks, and the server keeps running.

How to fix it:

  1. Identify the mod: the one responsible is the mod that reads in the physics properties, not the mods whose blocks appear in the list.
  2. Bring this mod under Mods to a version that matches your modpack.
  3. If you customized its configuration yourself, create a backup, restore the default state, and then apply your changes one at a time.
  4. Restart the server and check whether the message has disappeared.

Recipe refers to an unknown recipe type

What the message looks like:

Parsing error loading recipe <mod>:<rezept>: com.google.gson.JsonSyntaxException: Invalid or unsupported recipe type '<mod-id>:<recipe type>'

What's behind it: A recipe type defines how something is made, for example at the crafting table or in a mod's machine. This recipe uses a type that no loaded mod knows: the mod is missing, or no longer knows the type in its current version. Minecraft skips the recipe, and the server keeps running.

How to fix it:

  1. Read the mod ID before the colon in recipe type '…'. That is the mod that should provide the type.
  2. If this mod is missing and you need the recipe, install it under Mods.
  3. If it is installed, bring it and the mod or datapack containing the recipe to matching versions.
  4. If the recipe comes from your own datapack under world/datapacks/, you can remove it there. Restart afterward.

Good to know: If nothing is missing in the game, the skipped recipe doesn't affect operation.

Fabric mod refers to an unknown tag

What the message looks like:

Unknown tag: net.minecraft.class_<Number>@<Identifier>

What's behind it: A tag is a named group of blocks, items, or other content, for example "all logs". Mods use tags to recognize content from other mods. Here, a mod expects a tag that this Minecraft or mod version hasn't created. The affected content may be missing in the game.

How to fix it:

  1. The line itself doesn't name a mod; class_<Number> is an internal name from Minecraft. Look at the lines around the message in logs/latest.log. The first line with a mod name names the culprit.
  2. Update this mod and the Fabric API under Mods to versions that match your Minecraft version and each other.
  3. Restart the server and check whether the message has disappeared.

Good to know: How the versions fit together is shown in Fabric Loader and Fabric API: Resolving version errors.

A mod's mixin companion plugin can't be loaded

What the message looks like:

Error loading companion plugin class [<Class>] for mixin config [<mod-id>.mixins.json]

What's behind it: Many mods control their mixins, meaning their interventions in the Minecraft program, through a companion class. It decides which changes are applied. This class couldn't be loaded, typically because the mod doesn't match other mods or your version. Parts of the mod may then not work.

How to fix it:

  1. Read the mod name from the mixin file (<mod-id>.mixins.json). The package name of the class in square brackets also usually contains the mod's name.
  2. Update the mod under Mods to match your modpack version.
  3. If the message persists, remove the mod as a test or report the error to the author with the full message from logs/latest.log.
  4. Restart the server.

Good to know: If a feature of this mod is missing after startup, this message is the most likely explanation.

Archive (.zip, .rar, .7z) in the mods folder

What the message looks like:

Error for file:///home/container/mods/<File>.zip: java.nio.file.ProviderNotFoundException

What's behind it: The mods/ folder contains a packed archive, such as a backup of the folder or a downloaded package. The loader tries to read every file there as a mod and reports an error for the archive. /home/container/ is the main folder of your server, as shown in Files.

How to fix it:

  1. Under Files, open the mods/ folder.
  2. Move the file from the message out of the folder, for example to the main folder. If you don't need it, you can delete it.
  3. If you wanted to install a whole modpack, use the Modpack section for that.
  4. Restart the server.

Good to know: Only mod files (.jar) belong in mods/. It's better to create copies of the folder via Backups. Basics: Installing Minecraft mods and plugins.

Reference to an unknown entity type

What the message looks like:

Error: <mod-id>:<name> is not a valid entity ID!
Invalid or unknown entity type '<mod-id>:<name>'

What's behind it: Entity is the umbrella term for mobs, animals, and other movable objects in the world. A configuration, datapack, or command names an entity type, for example ars_nouveau:wilden_hunter, that doesn't exist on this server. The corresponding mod is missing or has changed the name. The entry is ignored, and the server keeps running.

How to fix it:

  1. Read the part before the colon: it names the mod the entity belongs to.
  2. If this mod is missing and the entity should appear in the game, install it under Mods.
  3. If you removed the mod on purpose or the entity has since been renamed, search for the name in the configurations under config/, in datapacks under world/datapacks/, or in functions (.mcfunction files).
  4. Create a backup, correct or delete the entry, and restart.

Mod is corrupted or doesn't match the version

What the message looks like:

java.lang.VerifyError: <Details>
EntrypointException: Exception while loading entries for entrypoint '<Name>'
The mixin '<Class>' is missing an @Mixin annotation

What's behind it: A mod can't be loaded properly. Its program is faulty, built for a different Minecraft or loader version, or doesn't get along with another mod. Several error patterns hide behind this message, such as a VerifyError (Java rejects a loaded piece of the program) or a failed entrypoint, the starting point of a Fabric mod. The affected part is missing, and in the worst case the server doesn't start.

How to fix it:

  1. Find the mod: it appears in the message itself or as a package or folder name in the stack trace below.
  2. Check whether all mods belong to the same Minecraft version and the same loader. A Fabric mod doesn't simply run on Forge or NeoForge, and vice versa.
  3. Update the mod under Mods or remove it as a test and restart.
  4. If the server starts without it, look for a matching build or do without the mod.

Good to know: Book … has use_resource_pack set to false concerns guidebooks that run through the Patchouli mod: the mod with the book doesn't match the installed Patchouli version. In detail: Setting up Minecraft mods and modpacks correctly.

Recipe with too many fluid ingredients

What the message looks like:

Recipe has more fluid inputs (<Count>) than supported (<Count>).

What's behind it: A recipe from a datapack or a mod integration requires more fluids as ingredients than the machine in this mod version can process. The recipe is skipped, and the server keeps running. Usually it was written for a different version of the mod with the machine.

How to fix it:

  1. Look at the lines around the message in logs/latest.log. If a recipe ID in the format <mod>:<name> appears there, the part before the colon shows where the recipe comes from.
  2. Bring the mod with the machine and the mod or datapack with the recipe to matching versions under Mods.
  3. If the recipe comes from your own datapack, remove it there or adjust it.
  4. Restart the server.

Good to know: If you don't need the recipe, the message doesn't affect the server.

Slice and Dice: recipe with too many fluids

What the message looks like:

java.lang.IllegalArgumentException: too many fluid ingredients

What's behind it: The Slice and Dice mod automatically converts cooking recipes from other mods, for example from Farmer's Delight, into Create recipes. At least one such recipe contains more fluids than the conversion supports. As a result, individual converted recipes may be missing. The server keeps running.

How to fix it:

  1. Under Mods, check which versions of Slice and Dice, Create, and the cooking mods involved are installed.
  2. Bring them to matching, current versions. The Slice and Dice page states which versions it works with.
  3. Restart the server and check whether the message has disappeared.

Good to know: If you're not missing any needed recipes as a result, you can ignore the message.

Better Caves cannot generate cave fluid regions correctly

What the message looks like:

Failed to fetch the AquiferContext.
This is a mod compatibility issue. Please report it to the Better Caves GitHub issue tracker!

What's behind it: YUNG's Better Caves needs additional world-generation data for underground water and lava regions. This data is called the AquiferContext and is missing where the mod expects it. The mod reports a compatibility issue, so newly generated fluid regions may look wrong. The server keeps running.

How to fix it:

  1. Create a world backup under Backups before changing the mod setup.
  2. Under Mods, check that Better Caves and its required libraries match your Minecraft version and loader. Check compatibility with other world-generation mods too.
  3. Test a corrected setup on a copy of the world first and generate new areas there.
  4. If the message persists, send the Better Caves author the complete logs/latest.log and your mod list. Mention whether terrain was being generated in advance.

Good to know: Changing the mods does not automatically regenerate areas already saved in the world.

Curios: invalid operation in a slot definition

What the message looks like:

Parsing error loading curio slot <Mod>:<Slot>
java.lang.IllegalArgumentException: set is not a valid operation!

What's behind it: Curios manages additional equipment slots. A mod or data pack supplies a slot definition containing an operation that your Curios version does not understand. In this example, it rejects “set”. The affected definition does not load correctly, so its equipment slots may be missing or configured incorrectly.

How to fix it:

  1. Read the complete identifier after “curio slot” in the Console or open logs/latest.log under Files.
  2. Identify the mod or data pack that provides this definition. Compare its supported Curios version with the release installed under Mods.
  3. Install releases that work together. If you edited the definition yourself, correct the operation using the documentation for your Curios version.
  4. Restart the server and check that the message disappears and the affected equipment slots work again.

Good to know: Back up custom changes before replacing mods or data packs.

Your message isn't listed?

Read the whole message in logs/latest.log, including the lines before it: the first error is usually the cause, and later ones are often just consequences. You can recognize the mod involved by the folder and package names in the stack trace or by a file name ending in .jar. Check whether it matches your Minecraft and loader version, and create a backup before every change. Errors that only occur while the game is running are covered in Plugin or mod throws errors during operation. If you get stuck, open a ticket and include the time of the message.