How to fix Time Machine claiming not being encrypted anymore? 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?
I deleted something equivalent to "2019-08-04-xxxxxx.inProgress" but that didn't work. Then I found another suggestion that did work for me:


Select Disk in Time Machine settings,  
Remove the disk, 
Re-add it