Apparently Calibre has a problem with Ubuntu 20: https://bugs.launchpad.net/ubuntu/+source/calibre/+bug/1884295
This problem should be fixed in Calibre, but it seems that apt install calibre-bin does not fetch this fixed version yet.
So I simply enforce the build to use an elder version of Ubuntu which fixes the problem at the moment.
Apparently Calibre has a problem with Ubuntu 20: https://bugs.launchpad.net/ubuntu/+source/calibre/+bug/1884295 This problem should be fixed in Calibre, but it seems that
apt install calibre-bin
does not fetch this fixed version yet. So I simply enforce the build to use an elder version of Ubuntu which fixes the problem at the moment.