What I did:
1. tested the Ajax Comment Form v1.01b did not work, so disabled it
2. under Stats > Database clean-up operations I did 4 operations (recount posts, reindex content, recalculate user points, refill event streams).
Then I navigated back to Admin > General, and there the options regarding URL structure disappeared! I only see the radiobuttons but no labels.
I don't know if my actions 1. and 2. led to this error, but just wanted to let you know...
It was something else, sorry!
I was disabling the $neatoptions[$rawoption] as discussed here. That was causing this bug!