You use this procedure or own backup commands ? ( https://docs.oracle.com/cd/E64076_01/E64083/html/vmadm-backup-restore-ovmm.html )
This error looks like bug: https://bugs.mysql.com/bug.php?id=87723 but I not see Oracle's notes for this problem and OVM
I did use the command from this note Oracle VM Manager MySQL Daily Backup Not Running (Doc ID 2365231.1)
Which is the same from Oracle's documentation.
I guess latest OVM Manager release comes with this bug.
Its better you raise a service request with Oracle.
And update the SR number here.?
So we can look into it.