There is a bad command for multiverse that ruins the server.
There has been a person throwing
/mv _REDACTED_
and server instantly crashes after that.

Join the discord or download the latest version from spigot. The issue should be resolved with those builds

Duplicate of #2703.

For those using older multiverse builds and can't update for whatever reason: https://www.spigotmc.org/resources/multiverse-patches.96390/