That's what I did first, because there was another error popping up with the river mod activated. That already happened when the seed is generating (well, it did'nt) and since the error message had the word "river", I deactivated it.
So while trying to load the anomien biome, the river mod IS turned off^^
Deactivated all mods except anorien. Worked.
long story short:
It's popping up again when the desert transition mod is activated, too^^
I will test now if it's working with all other mods
It works! - Just too bad I can't make the river mod working But nvm, it's fine now.