Guest Operations - The operation is not supported

Hi,

I'm using vmware.vim api in a C# project. When I try to get GuestAuthentication object with AcquireCredentialIsGuest I get the following error:

"A server fault of type NotSupported with message 'The operation is not supported on the object.' occured while performing requested operation."

Guest Commands aren't disabled, neither on virtualmachine or on host. I'm running out of ideas what could be wrong. Can anyone help me out ?

Code:

GuestOperationsManager t = (GuestOperationsManager)client.GetView(client.ServiceContent.GuestOperationsManager, null);

GuestAuthManager c = (GuestAuthManager) client.GetView(t.AuthManager, null);

NamePasswordAuthentication userCred = new NamePasswordAuthentication();

userCred.Username = "xxxx";
userCred.Password = "xxxx";
userCred.InteractiveSession = false;

GuestAuthentication a = c.AcquireCredentialsInGuest(vm.MoRef, userCred, null);

Thanks!