Today, after numerous tests, it became clear that:
1. Even in the most primitive build 1.15.2 with Multiverse-Core plug-ins, whether AdminCMD Essentials this error can not be avoided by more than one server even with a paper core

Multiverse-Portals-4.1.1-SNAPSHOT.jar
Multiverse-Core-4.1.1-SNAPSHOT.jar
EssentialsXSpawn-2.17.2.0.jar
EssentialsXChat-2.17.2.0.jar
EssentialsX-2.17.2.0.jar
 
The error occurs when more than one world is created on the server
  You begin to pump a resident with the profession of Cartographer
When you reach the resident level master and sell the resident a glass panel, the client freezes
The server also hangs and gives an error Spigot Watchdog Thread/ERROR

Please forgive my bad English.