Forge version: 2707
Twilight Forest version: 1.12.2 3.7.424
Link to crash log: https://pastebin.com/57xeAzCm

Steps to reproduce:

1. Logout when in Twilight Forest Dim
2. Try to log back in. Screen stays on Loggin in...

What I expected to happen: I should be able to login

What happened instead: It won't let me, even though the server thinks I am about to.

Can you also provide the client log when this happens.

Sure
https://pastebin.com/bGt3stZQ

Looks like it fails here:
NullPointerException
mcjty.lostcities.dimensions.world.LostWorldType.getBiomeProvider(LostWorldType.java:65)

Thanks

Seems to be fixed: McJtyMods/LostCities#136 (comment)