Fluidlogged-API and other change fluid behavior will cause this crash, the developers of Angry-Pixel has not responded yet, just report here that let you to look about it and mark it to here, the issue can find aat here:
Angry-Pixel/The-Betweenlands#1343

it fixed in the dev.

SmoothWater isn't needed if Fluidlogged-API is installed.

It just only use Fluidlogged-API or SmoothWater such one mod with Betweenlands and get crash, this is the error on the Betweenlands side, not here. just mark this issue to here.

Try running the game without Fluidlogged API (just to make sure the issue is in fact caused by this mod), I tried running just Fluidlogged API with Betweenlands v3.9.1 and things worked fine. Also don't use Smoothwater alongside MAGE, since Smoothwater is just a deprecated version of MAGE with less features.