My MacBook (10.13.6) is backing up via Time Machine to two separate NAS (Synology). This has worked fine for almost a year now, having Time Machine selecting one destination after the other.
Since yesterday however I have the problem, that the backup to one NAS can't be finished. The message is:
Time Machine did not back up because the backup disk was previously encrypted but is no longer encrypted.
The backups have always been encrypted and I didn't change this. Only thing I can imagine is that it has something to do with an update from 10.13.5 to 10.13.6. I think I did this after the last successful backup (as it's stated in the Time Machine dialog).
Peaking into the
MyMac.sparsebundle folder on the NAS I see a difference here:
The folder with still working backup has a file
token of size 119.5 KB while the one from the broken backup has 0 size.
How to fix this?