I used an online website to generate a custom "/summon" command (minecraft.tools) and copy and pasted.

It spawns, but shortly after the game freezes. I was using repeating clocks to add scores to my custom mob, but I've tried several different custom mobs and they all crash.

EDIT: Most of the time when I force quitted the game, the last thing in the log would simply just be "Stopped server"

https://www.youtube.com/watch?v=i5j6I0cnD2Y - Video of Bug