I'm getting this exception when I connect to the server (joining the world). The world I connect to is the main overworld (nether and end are disabled). As result the server crashes.
I optimized the world using the built-in mechanic by minecraft.
The exception does not occur if no player connects.
Crash-Report: https://pastebin.com/5WPecuZz
Full-Serverlog: https://pastebin.com/9Gp9K67k
I managed to figure out why that exactly happens: I accidentally called World#close() by using the Resource-Try-Catch for the world in the WorldInitEvent. Removing this fixed the problem and the other exception occurring right after the server is fully started.
For that: can be closed.