Ok, problem ist solved. I reverted to the older version 3.48 as shown on the homepage of calibre.
https://calibre-ebook.com/download_linux
Everything worked fine with every version of calibre and it is working fine now. Thanks a lot for the help to everyone! ;-)
I saw too late that the master is answering... ;-) Thanks for your help Kovid and for this great piece of software I am using for years now without problems.
regards,
Karsten