For some reasons I still do not fully understand, my OMV set-up neither could be reached over LAN nor was booting properly.
I therefore ...
- connected the SDD with the backups to a laptop running Ubuntu,
- downloaded the newest version of etcher (since I was on an Ubuntu machine, I used Linux 64Bit appimage),
- flashed the latest full backup (ddfull) to my spare sd card,
- removed the power cable from my pi,
- swapped the sd card with the one with the newly flashed backup,
- powered up the pi and volia: everything is back to normal!!!
I'm not sure why etcher now worked flawlessly compared to my dry-run in April 2020. Last try, I was using etcher on a Windows PC and now on Ubuntu. Not sure whether that was the reason. Anyways, usbimager is available on various systems (Windows, Linux, etc.).
Many many thanks for the backup plugin: it definitely saved my say.