This happend to me 2x.

If I connect a mob with a lead to a fence and then restart the game is causing a ticking entity which crashes my world.
The first time it happend to me was with a wandering trader llama, the second time with a normal chicken.

I guess it is caused because of the connection from the lead to fence.
After I deleted the entity with NBTExplorer, I could load the world with only one problem.
The problem was that wooden pressure plates do not reconize my character. After using F3 + A it worked again.

No mods used.