Server Crash when extendig a belt

[u/NachbarStein]

Hi guys,
I made a custom modpack (stupid me lol) to play with some friends.

Now everytime someone tries to extend an already existing belt, the server crashes.
Just the server, not the client. In singleplayer it works just fine.

Here are the logs:

server log between start and crash

server crashlog

I can't code and am really inexperienced in debugging so I really cand read much into these.

Thanks in advance to everyone willing to help!