Any suggestions what might cause the described problem?
I'd look at the "/etc/udev/rules.d/70-persistent-net.rules" file in the Guest (Ubuntu Server) and make sure each device has a unique MAC Address. Personally, I'd just delete the file and reboot the Virtual Machine as it will be (properly) recreated automatically and will rule that out as the (possible) cause of the issue.
Is there still something missing on consfiguration side?
Not that I can readily see.