I suppose I accidentally changed a settings but I have no clue where and when.
I do not know how it is called in english therefore I try to describe it and show it with a picture.
Please help me.
the problem: when I select a viewport and move the view, the objects suddenly are displayed in a kind of wireframe instead of normal. On the picture the trange view is the right upper. Please tell me step by step where I can change this setting to normal.
Rightclick in the upperleft cornerpanel. You’ll see the list of display mode options.
I suspect you checked the box for BoundingBox display:
John, THANKS!!!
It’s a handy tool if you have a big file and a display adapter with poor performance.
But very unhandy 
when accidently set and not knowing what you did.
Thanks again for your help.
I was already looking for 3 days how to solve this… 