Failed to get cluster nodes

I just hit the same error, failure to get the list of nodes.  My VBR server is not domain joined, but it uses a serviceacct@domain.com master account.  It is NOT a domain admin, but a member of the Backup Administrators group.  I explicitly added the Backup Administrators with full control to the AD computer objects for the cluster and the nodes (not sure if this was necessary), and then I added the service account to the local administrators group for the two cluster nodes.  At that point the credential test succeeded.