Like @tpnsolutions already reported: After a very extensive research and no solution in sight, a more pragmatic approach was needed. We spun up a new (pretty much identical) KVM, installed Virtualmin as per documentation and started doing some testing. Those showed no sign of malfunction.
I then carried on to migrate the rest of the servers data (web/mysql and email) and modifying the DNS records as things progressed. Today I can report, that everything is in working order and things are running smoothly.
One little niggle did develop during the transfer in that Yahoo decided to block my email traffic. I have been in contact with their support team and are hopeful that this glitch will be corrected soon.
Lessons learned:
Even though that the Virtualmin backup/restore process is pretty spot on for itās intended purpose, using to migrate from one OS to another may need a bit more of a hands on process. āTake it slow and do a step-by step approachā would be the generic adviceā¦
Thanks to the team āmakingā Virtualmin and especially to Peter for having the stamina to help me see this project getting through. I certainly learned a lot more about Virtualmin in doing so!