Hello, so I noticed that sometimes the game will freeze for a few seconds and crash and the report mentions the following:
java.lang.ClassCastException: net.minecraft.entity.monster.EntitySlime cannot be cast to net.minecraft.entity.EntityCreature

So I tried to spawn a slime with a spawn egg and it did not spawn anything and the same message appeared in the console but it didn't crash. I guess it only crashes when they spawn naturally? idk. Magma cubes and modded slimes also wouldn't spawn, so I guess it has something to with that type of mob. It would spit out a similar message but with MagmaCube or the name of the modded slime in there.

And yes, I tested without any other mods installed.

Will investigate.