Describe the crash
The server crashed when generating the Nether for the first time, when returning back from it the first time, and also when just loading into it the time after that.
After talking to a staff member at Akliz, I think they told me it was due to (or probably due to) the BetterNether mod, and some issue with the world seed.

To Reproduce
Travel to the nether for the first time on a server. Also then travel back and forth a few times after that.

Crash Log
crash-2021-01-26_02.44.30-server.txt

crash-2021-01-26_02.52.38-server.txt

crash-2021-01-28_21.16.42-server.txt

Info (please complete the following information):

• Modpack Version: 3.0 Beta 3
• Server or Singleplayer: Akliz Server
• Version the world was created on: 3.0 Beta 3

Additional context
The world was initially single-player, hosted on LAN (this is all way before we went to the nether), but when we moved it onto the Akliz server, for the first evening we needed to go beta to Beta 2 since Akliz didn't have 3 yet.
But when we went to the nether, we had been back on Beta 3 for about a week.

It shouldn't crash anymore next update, but the insane lag will always be there unless a modder decides to fork better nether or we figure out a different implementation for nether cities