www.ntgb.at/impressum
if i set the error description as shown in the screenshot, the modules don't have anything to write in, which means the title is displayed but the log in form for example is not displayed. in case of the search the module title is shown but the textfield where to write the words in is not shown. so i can't log in nor search for anything because i can't write anything.

Then please try the setting = Simple

But the error reporting level has nothing todo with your issue

I have already tried all different settings...
what can it then be ???

I have fixed this issue on clients server, it is a Akeeba bug.
I have disabled the Akeeba installation function and then all works fine.