I also saw this error, this was in 6.2.1, in an environment I inherited. The problem was that under Storage Systems in VSC they were SVMs.
Once I added the cluster IP/admin account the backups worked.
As you would expect, the admin passord changed and the backups started failing again, I know there's no surprise there.
So we just match the password in VVSC to the new password and the backups ran.
We are running ONTAP 9.2P2, vSphere 5.5.3 and VSC 6.2.1P1D5