Hi!
There is a crash when I go to the Nether. Without Locks mod this doesn't happen.
https://pastebin.com/5i7fG2pK

So did it work?
If yes you can close this if no, we can try to make it work.

Hello,

In server.properties try setting max-tick-time to -1.

By default, it is max-tick-time=60000 which mean the max tick time is 60 second more than that it crashes, it means that when a player join it sometime create a lag spike that is so big that it crashes the server.
By entering the value -1 it disables the limit.

My best advice is to search for some optimization mod or get a better server.

Oh. :D Okay, I'll try.