Exception ticking world: Non [a-z0-9/._-] character in path of location: minecraft:Zombie

Just playing in a world created before 18w14b, when it unexpectedly crashes. Seems to have to do with a zombie - I can get back in the world but I'm not fast enough to paste the /kill command to get rid of it, and time until crash gets shorter each time i join the world.

EDIT: The difficulty is on Peaceful - there are no zombies.