I have seen this for the past few times i have tried to use this feature. both in 16.04 and 18.04 LTS versions. I will go into the restore backup area..fill in the ssh information...point it at the file and it says this is not a valid backup file. I will then log out of virt....log back in..put in the exact same information and it works the second time.
backups fail on a routine basis