I finally did it!
I opened the "sample" file and deleted all walls and interriour. Then I wanted to delete the "background" picture - editting was possible, but no opening of a new one. So I deleted the background file...
Tried to "open" a new file - tada! "Open" dialog appeared, and from that time I was able to open, save and safe-as any file!
Finally it worked, no clue why, but now it is fine again!

By the way, I also tried the line you posted, no difference - no log-file was created.

Tzhanks for your support!!