Oh, I misunderstood your last message, it was the FILE backups that was working, my bad.
Ok, so the image backups are per partition, not per device it seems, maybe.
What I would do is try to do an image backup on a disk with only ext4.
I remember briefly looking into img backups and reading first that it wasn’t supported, but then a while ago experimental features were introduced or something.
I honestly have no more input to give you.
Edit
I found where I read the C = /, it was a blog post from 2 years ago. It’s mentioned doing it without the dattodb…
The blog post invites discussion on this forum, maybe try that. xD
Have a look.
Edit 2
You might want to disregard most of my message and read this.