More shocking, it took me 3 days (evenings that is, since during the day, I do perform some sort of day-job).
To be learned from that: change the HDD/SDD before attempting to upgrade. A strategy I was doing before, which never was needed... now that I did not follow it, it was :-(
Anyway, I had my backup... and after some failing attempts, I got my backup restored.
The HP ProBook is back to being a HackBookPro :-)