Thorben,
How are you getting the book on your Kindle device? Certain methods are known to break the EPUB. Could this be related to this issue?
To emulate what the reader will consume, you need to either copy the file to the Kindle device directly or publish it through KDP (in draft mode obviously).
The more detail you provide about your steps, the better equipped I will be to advise you.
Cheers,
-Dan