login credential issue on vcenter 5.1

Hey guys,

Recently I upgraded my vcenter server from 4.1 to 5.1.  Orginally (4.1 version ) I use my AD account to login without the domain name prefix, just like username and then password.

But in vcenter 5.1 I can no longer use that in simple way, seems it must use the domain\username such of type.

Does anyone have some experience on it?  Any idea to change back ?

Thanks.