I have no idea what that means. I was jumping around on half slabs and flying around with an elytra in a world from 20w22a, and sometimes the game just crashes. It usually happens when I am on the edge of a block, and I guess it just doesn't know where I am?

I checked. The position in the crash is not where my player is. It's also strange because I have a platform at Y=63.5, so Y=64 is not possible

The game crashed whilst ticking entity
Error: java.lang.IllegalStateException: POI never registered at ft{x=-238, y=64, z=-165}