In single player I broke a sign (that had a waystone assigned to it) with my empty hand and it crashed and I landed in the multiplayer menu. Upon re-entering the single player world it no longer allows entry and does the same again. NOT easy to reproduce.

EDIT: Starting another world I can't seem to reproduce it now. o.o

That's strange... Do you still have the error log?

No report was generated. So strange. Also, upon messing around with the other world trying to reproduce it the original world is fine now. eh... I have no clue here

EDIT: Just breaking a bunch of them in a row in Creative (had all the different kinds out there) caused a perm-a-freeze as well but no log. :/ I noticed the Iron version had a "no texture" black/purple animation when breaking it as a side note.

Problem found and solved :D
The next update should fix it.