Maybe one step in the direction for the solution:
Today I added a book (epub) and I got a result from amazon_de.
I looked at the url and I realised that the site had the "old" amazon style.
For you to look at it, if you want to:
Result "old style" (Amazon uses this for the Kindle Edition):
http://www.amazon.de/Vollmeisen-Roma...=UTF8&qid=&sr=
"new style" (Amazon uses this for the pocketbook):
http://www.amazon.de/Vollmeisen-Roma...mm_pap_title_0
Is it possible, that this is causing my problems? And if so, do you have an idea, how I can solve the problem?
Edit: the problem exists for me even in 2.39