thank you for this hint. But this was not the problem. Nevertheless you are right that two overlapping walls do exactly the same effect. I had this before and figured out myself this issue.
In the case here, I checked this, but definitely just one wall.
Unfortunately I can not post the sh3d-file, because I continued to work on this w/o having saved this situation.
Oddly enough that the effect has gone away by continuing the work (see attached picture).
If I see the problem again I will post the sh3d-file.
