I’d highly recommend spinning up a new, clean Ubuntu 24.04 instance, installing Virtualmin using the virtualmin-install.sh script, and migrating to the new server rather than spending days trying to fix a bug that very well may not even be ours.
Besides, I’m pretty sure there will be many more various issues that will suck a lot of time. It simply isn’t worth it. That said, I imply a production-like server, which should be clean and predictable, with minimal history.
However, if the goal is to play around and find the source of this fancy bug, then it may make sense, and we can go on then …
The system was upgraded over a series of years, usually a few months after the release I was upgrading to. It was a simple bump forward with do-release-upgrade, which I did couple months ago.
The one and only problem the system has since that upgrade, is with upgrading Virtualmin, and that only recently – I did an ordinary apt update; apt upgrade a few of weeks ago that went fine; then went to update again last week, ran into this issue, and attempted to get support for it.