Back on April 29th we posted an alert: ALERT: Login issue after updating to vCenter 5.1 Update 1 which detailed a scenario whereby customers might be unable to log in using the vSphere Web Client or domain username/password credentials via the vSphere Client after updating to vCenter 5.1

Tonight at 7:30pm PST, vCenter Server 5.1 Update 1 will be removed from the VMware download site and will be replaced by vCenter Server 5.1 Update 1a. The primary aim of the 5.1 U1a release is to address the regression that was identified in 5.1 U1.

--> Please see the rest on the blog : http://blogs.vmware.com/kb/2013/05/vcenter-server-5-1-update-1a.html