Minecraft runs without problems when the world is first created
went to end and better end biomes load normally
but when I log out of the game I can never enter the same world again until deleting the better end mod

Modpack - All the mods 6 : v1.39
Same error occurs in versions:
betterendforge-1.16.4-1.2.0.jar
betterendforge-1.16.4-1.1.0.jar

• Some Extra mods (all mods listed included in logs bellow)
  Initial log opening Minecraft
  https://paste.ee/p/NwY7c

Log when trying to enter world save
https://paste.ee/p/FclK4

Main log
https://pastebin.com/Ji4kWnsD

that's awesome, I am curious if you don't mind me asking, what was the issue ?

Yeah, i remember you already told me about the issue with All the mods 6 pack. So, after someone else reported the same problem with another list of mods some days ago, i finally managed to spot and (hopefully) solve that subtle bug. In the next version (coming tomorrow probably) you should be able to play without this issue!

I forgot to add BetterEnd structures to the FlatGenerationSettings structure list, a very subtle issue actually, which normally doesn't cause problems..until another mod uses that list in someway!

New version out, the issue should be fixed so i'll close this. In case the issue shows up again open a new issue please