Hi,
Are you facing the situation after upgrading to the 5.1.0 U1b...? As per the KB 2036529 The fix is for a future release if you are running on a existing version. If you started to face the issue after upgrading to the 5.1.b then the workaround is available in this VMware KB: AD users with customized UPN user names cannot log into vCenter Server after upgrade to vSphere 5.1.b .