On recommendation of the DVDFab support, i went down with the Kindle Desktop to 2.7.1, updated BookFab to 1.2.0.9 and redownloaded the book. With this combination the conversion worked. Many thanks to the support team for the help.
Quirken
Great! We're glad to hear that solution worked. Please continue using this method for now.
Our developers are actively working on a fix for the compatibility issue with Kindle for PC v2.8.