This issue seems to only be happening to certain players, as some of them are able to join the server with no issues.

Server thread/ERROR [ne.mi.ev.EventBus/EVENTBUS]: Exception caught during firing event: null

This is the log from the server for whenever one of the players attempt to join, and are unable to:
https://gist.github.com/Ridanisaurus/f3f53fd169f3c3c74f370b71aca05888

Can you add your full server log (maybe even debug) for the sake of completeness as well? This issue makes no sense to me unfortunately

Logs: https://gist.github.com/Ridanisaurus/279bc6a5788d07e7f1d1849f512fcb9e
Debug: https://gist.github.com/Ridanisaurus/841bd7a52d87a2b9c504c25fa4014d46 (the original Debug was 537931 lines, so I had to remove all the log spam)

I unfortunately don't have a reliable way to reproduce this issue myself, however, better error handling is going to be something on our radar for the near future, so if this is indeed a downstream mod erroring that would probably help that at least

Getting the exact same issue, can upload logs but they are very similar to what's already there.
I'm able to join the server fine, and so is my brother, but my girlfriend using the same pc and installation as my brother cannot join.

I am by the way using version [1.30.55]