I've started a new server running Mowzie's Mobs recently, and while testing I found that the client (not server) will crash if switching to 3rd person while using the Ice Crystal. Crash log can be found here.
Let me know if more info is needed! (Minecraft Forge 1.16.5)

Update: Also seems to happen with the Sunbeam attack

So strange. I could've sworn I fixed this already but your log says it's up to date. Could be a compat issue. Does this still happen when Mowzie's Mobs and Geckolib are the only mods installed?

I'll give that a test ASAP tomorrow morning and let you know- Also I should mention I tried disabling custom animations in the Mowzie's Mobs client config, and that seemed to prevent the issue from happening too.