Not too sure how many of you ended up in the same boat, but we went with the early days of R80 mgmt and then later upgraded to R80.10 using CPUSE.
Backups stopped working for us on R80.10 failing at very last stage of moving mdsbackup.tgz archive to the final backup_host_date.tgz.
Case sat with CP for a month or so but we got nowhere. Until I had bit more time to play with it in the lab.
Turned out that R80 > R80.10 upgrade messed up backup schemes, see green one below is from VM that was created fresh with R80.10
and here's one from VM that was upgraded from R80 to R80.10
Simple solution - remove all the "extra" junk leaving as per green screenshot and it will work like a charm again.
In case someone else ended up with the same issue.