I'm not good at being overly verbose
... no, actually the problem was my stupidity: the timestamps of the files were clearly different, a clear indication that these files were different backups.
With that said, I ordered the exact same SD card. Shipping took some time but I can now test whether it is possible to restore the full-disk backup to a new SD card. The outcome:
- I cannot restore the backup with etcher (failed three times)
- (I haven't tested w32 diskimager)
- I manged to restore the backup with usbimager on the first try. Can boot from the restored SD card without any problems any everything looks fine
(formated the SD card with SD Card Formater between each try).
I don't know why I could not restore the backup with Etcher. The error message was not really revealing ("failed device / write error"). No idea what the problem was.
I can write a guide if this is helpful.