(I posted this as a comment on another post but that post was really old so I decided to just make a new one here). The crash seems to happen when the rocky roller and the bear (any variant) collide with each other. I'm not exactly sure why or which mod is causing the other one to break to the point of crashing the game but the easiest way to recreate this bug is as followed; download alexs mobs and untamed wilds for 1.20.1, and open a new game (a superflat creative world is fine for testing this). after that either; dig a 3x3 hole and spawn a rocky roller in the middle of the hole then spawn a bear on top of it. or; spawn a bear and a rocky roller then go into survival mode while the bear is between you and the rocky roller and then go into creative mode again so they fight, that will eventually cause (what I believe to be) the same crash. Here is the first crash I got in case you did not see my original comment

Can confirm, same thing is happening to me. I'm gonna get ChatGPT to make a datapack that will stop them fighting eachother and see what happens