Forget my previous reply, I thought you were doing PowerCLI cmdlets on the guest.
Is the account under which you run the script known on the guest OS ?
Did you try a logon with that account in the guest OS ?
And did you try to execute that PowerShell line inside the guest OS ?
It might be that the account is not set up correctly inside the guest OS.
Or that the account can't even execute PowerShell code inside the guest OS