A plugin touched the world from the wrong thread
Applies to Paper, Vanilla.
What it looks like in your log
java.lang.IllegalStateException: Asynchronous entity add!
What causes it
A plugin modified the world from an async thread, which the server refuses to allow. This kicks players or crashes the server.
How to fix it
The stack trace below the error names the plugin. Update it, and if the error persists remove it and report the bug to its author.
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