[mtbopfuyn]

How long was he doing his business for the lock to freeze? He couldn't crawl under the stall or break through the door?

[dirtboy]

How long was he doing his business for the lock to freeze?

There could have been some super-cooled water in the lock, and when he disturbed the lock the water flash-froze.

[Welsh Rabbit]

He couldn't crawl under the stall or break through the door?

I haven't been to Germany since the late '80s, but as I recall, the doors on a lot of public toilet stalls were full doors. Many were also pay toilets as well, so the locks were a lot sturdier than the ones you find here.