Using my domain as default and using domain\user I get this error
The authentication server returned an unexpected error: ns0:RequestFailed: Group was not found. GroupSID= ''.. The error may be caused by a malfunctioning identity source.
but when using localos\root I can login
but now I have nob idea why this is not working with vCenter 5.5 and 5.1 is OK. (in 5.1 I don't need to use domain\user )