Server crash after join

So i have a problem with my Server,

yesterday everything was working fine nothing is lagging or crashing, but today at like 2 am the server suddenly stops and shuts itself down while we were exploring trying to find a monument.

so we waited till it stopped started the server i could join but when my frined tried to join it instantly stops and shows the following error on both of our screens:

Internal Exception: java.lang.StackOverflowError

and the log of course: https://mclo.gs/ddBFt6O

I am hoping someone can help us.

it only crashes when my friend joins i can join without any problems thats the issue we are having

Thank you,

the Server is now working fine without crashing.

I appreciate your help!