Problem is sovled: I did all steps as described in Ticket #16 and that works great - many thanks for the author who provided the step-by-step solution. Perhaps one important step was to deinstall all audible products and also remote the audible directories as explained in tickt #16. Thus this ticket can be closed as resolved :-)
I tried it on an other Notebook with Windows 10 but without update and the problem is similar: First the result was the same: The convertion directory was created but only the cover.jpg file was created and the error message came "AAXAuthenticateWin: -32". I authenticated the Audible App " Download Manager" and got it play my downloaded book (the aax file) and then calling aax2mp3 it got much further: It processed all chapters but didn't create any of the chapter files but again only the cover.jpg....
After Update of Windows 10 Error AAXAuthenticateWin: -27