Listen to Your Toolkit
For the PDF output avoid topics in the fourth order.
The short descriptions in some topics were indented incorrectly. Namely in those topics that were created on the fourth level.
I could try to find the error in the PDF2 plug. But that would cost me too much time. I think the toolkit wanted to give me a sign. Never structure your chapters deeper than 3 levels. And it is right. I decided to listen to the toolkit.
Now each output format (Ausgabeformat in German) has its own chapter heading in level 3 and everything is fine.