Thank you for your reply, Ayumi!

Unfortunatly nothing has worked out so far,
I have run the Map Compiler a couple of times,
here are the present results:

- I turned on the compiler warnings and let it create a logfile
- I have compiled with default settings (still crashes)
- I have set the "detail"-flag (still crashes)
- I have divided my blocks up in six groups (still crashes)
- I TRIED to uncheck the "concave"-flag but it automatically activates itself again

Now, the logfile/compiler warnings are a loooooong list of "Duplicate Plane"-errors, and also a small couple of "Vertex out of plane"-errors.
But the "Duplicate Plane"-errors are... humongously many!
The Manual says that two planes are the same or similar and I should delete the object, but it looks like almost all of my objects have this problem and I don't even know why two identical planes are a problem

Will run the compiler a few more times, but I think I can't solve this on my own