Craig was so kind and provided me with access to the VM images. With that image I was able to reproduce the upgrade failure. I checked the systemd state before the upgrade and noticed that sstemd-logind.service had failed to start due to dbus not being installed.
So I made a second test-upgrade and I installed the dbus package beforehand. With dbus being installed and dbus.service running, the dist-upgrade was successful. There might be an issue with the internal dbus socket, that systemd uses for communication. My guess is, that with dbus.service running, it can fall back to the system bus for communication in case there is something wrong with that internal dbus socket. -- Why is it that all of the instruments seeking intelligent life in the universe are pointed away from Earth?
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