https://pastebin.com/yh8sQ2Bx

How did you encounter this crash, and how would you reproduce it?

It just after world creation making me unable to join.
It does not happen to a saved game.

Can you send me your Aquaculture config?
My best guess is that you (Or whoever made the modpack) have messed with the config and accidently left some stuff in there twice - as I was able to reproduce it when doing that.

I recreated the world. Firstly, this crash appeared. https://pastebin.com/D89m5bKm
Then, this crash appeared: https://pastebin.com/NjHzBTip

config: https://pastebin.com/hD5Uvjpz

I've attempted a fix, could you try it out here? https://jenkins.girafi.dk/job/Team%20Metallurgy%20Mods/job/Aquaculture/job/Aquaculture%202%201.16/61/

The first crash still happens, but I don't think it has to do something with this mod.
Second crash is fixed and I can join. Seems to be fixed.

still throwing a nullpointer https://pastebin.com/4dguG7am

That's not a nullpointer, that's a ConcurrentModificationException (Also called CME).
Some mod seems to be messing with data loading on world join/creation, as stuff seems to be loaded more times than it should. (Was also the case with the previous issue, I just added a workaround on my end)
Here is an attempted workaround to this issue: https://jenkins.girafi.dk/job/Team%20Metallurgy%20Mods/job/Aquaculture/job/Aquaculture%202%201.16/62/

I would take a time to wait for this crash.
I'll be closing it for now and will reopen it if it will occur again,.

I'm running in this crash four times today trying to create a new world. That is very awkward, because it was recreation of my test world which I recreated multiple times before, without adding any mods or changing any configs.
https://pastebin.com/MXwTm2U3

Could you try and remove Enimatic Legacy, as that is mentioned in the crash too?

alos happens without https://pastebin.com/QXU5QU1t

Could you try and remove FTB Chunks? It has a Mixin that messes with something related to biomes.

I took it out and it hasn't appeared yet again, but the crash tends to appear randomly, so it might came up later.

I thought you were saying it appeared on world creation?

Yes, but it disappeared whilst I were testing and came back later.
I'll be do some testing again tomorrow.

Sounds good.
So the thing that is happening now, is that somehow the BiomeLoadingEvent is being loaded - while it's already being loaded (Basically just twice), causing the CME.

Closing as the last I've heard the issue have dissapeared. Please open a new issue, if it comes back.