Versions

Betweenlands version:

TheBetweenlands-3.3.4-universal

Forge version:

1.12.2 2635

Singleplayer or Multiplayer:

Multiplayer

Installed mods:

https://minecraft.curseforge.com/projects/ancient-towncraft

Link to full crash log

https://gist.github.com/ProsperCraft/89590669a6684a185cd86f00abaccb56

Description of the problem

How to reproduce:

1. Connect(server is in the modpack already)
2. • fail, fail, get in and get kicked, fail, fail, connect and get in.

I don't think this is specifically related to the Betweenlands because some vanilla code is also throwing exceptions in the log you sent, and whatever is happening shouldn't happen in the first place.
However, I've already added a workaround/fix to the dev version so it should be fixed in the next update.

A new update is out with the "workaround". I don't think it'll help, but it's worth a try