Hi,
Thanks a lot for your support.
At the moment I am using calibre 3.21 as that one is offered within the package management system of Ubuntu 18.04 LTS.
My findings:
- A new profile of Firefox 62 (moving .mozilla to eg. .mozilla.old) did not solve the problem.
- Setting "browser.cache.offline.insecure.enable = true" in the about:config page of Firefox 62 as suggested in the link above has solved the problem.
I also do not understand why anything has to be done by https even within a local domain but that's a philosophy of browsers in general so I am afraid that I will have to spend time in creating a https server.
Best regards and again many thanks,
Uli