Mods involved in the conflict:

ThaumicAugmentation-1.12.2-2.1.4
iceandfire-1.9.1 RoTN edition with some fixes, if relevant

I was flying on a creative world testing worldgen and ran into a crash with an ice and fire sea serpent bubble attack, disabling thaumic augmentation allows the world to load properly.

Here you have a crash report when enabled again:

crash-2021-10-11_12.40.06-server.txt