I'm having the same issue.
Did you ever find a solution to this?
At first please make sure you're using the correct credentials: if you're using domain make sure that Name field contains just name and you enter domain into Domain field (Those fields are in Credentials).
If you have more vCenter servers, do you the same credentials for all of them, or credentials are different for different vCenter server?
If you are sure that credentials are correct, please contact support as we'll need a bit more information.