Okay, in the end I tried everything and could not make it work.
As I however knew nothing is there/needed to be updated I went the way of adjusting the source code (in /var/www/html/lib/private/Updater.php) to ignore that error message and it successfully did the “upgrade” (it changed nothing as far as I could see).
Only then I could do the actual upgrade dumps of bumping +1 version etc. etc.
Thanks for that idea to this older forum thread.