tried to join with an invalid session — cause & fix

A player joined with a Java client that has no valid (purchased) Microsoft/Minecraft account and got rejected — your server has the premium check (online-mode=true) enabled. Less often, it's just an expired session token on an otherwise valid account. Your server itself is healthy and keeps running normally.

The standard way in is a purchased Minecraft: Java Edition account (Microsoft). If the message persists despite a valid account, restart the Minecraft client (expired session). Bedrock and console players can join instead via your server's crossplay address. If you deliberately want to allow players without a purchased account too (cracked/offline mode): without Mojang authentication anyone can log in under any name — that needs an auth stack (login plugin), which we set up automatically for you in cracked mode.