Found the cause of this nasty game breaker bug -- and it's a classic one :-/
The representation of the map size in my code is one field too small, which leads to a corner case in one of the numerous array operations. Together with my false cocky trust in my own coding abilities (which let to a missing condition) game made bzzzzzzt... in this far side of the map. I never walked there in my tests, because it is a dead end. Like so often in development a lot of unfortunate events finally connected into a crash.
Or to say it more poetic - by going all the way out there, you woke the demon ;-)