ProtocolLib does not match this Minecraft version
Applies to Paper, Vanilla.
What causes it
ProtocolLib, or a plugin built on it, predates the Minecraft version you run. Its internals reach into server code that moved.
How to fix it
Install the latest ProtocolLib build for your exact Minecraft version (dev builds after an update). If errors continue, the plugin using it needs updating too.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log