"Last automatic backup failed" but not indication why?

I have backup set to backup to local and OneDrive. It has been working, but today I’ve seen this message

However, there is no indication why? How do I debug this?

Have you checked it out on One Drive, also check the local area (samba is great for this to peer into HA. I would gather that any one location issue would trip an error. Just have to find out if which one or both and go from there.

Could be a space issue.

Where the backup worked, there are log files; but then of course, the backup worked. Where the backup failed, there’s nothing.