I’m confused. Are you trying to use dattobd on a btrfs? Not sure that is how it’s supposed to work, I think there is another snapshot mechanism available to use on btrfs when you install the client.
I have never done img backups on linux, that was the next thing I was going to learn so this is just me speculating.
In my notes I found this link referencing snapshots. Might be something to read?
If you like me, have multiple filesystems you back up from (I do file backups from btrfs, ext4 AND ntfs on the same client) this read could be good. But then again, this might not have anything to do if you do img backups.
And I don’t know what filesystem you store your backups in but this link could be good to read if it’s in btrfs.
Create the file /etc/urbackup/backupfolder containing the path to the backup directory for btrfs.
Test with urbackup_snapshot_helper test
https://www.urbackup.org/administration_manual.html#x1-11200011.7.2
Hope that can give you a little help even though I have not done img backups myself.