- You are unable to log in using the vSphere Web Client or domain username/password credentials via the vSphere Client.
This issue can occur if the specified vCenter Server login domain user account is associated with a large number of domain groups and multiple domains are configured as SSO identity sources. The precise number of groups at which this issue can occur varies due to the nature of Active Directory internals. However, it is more likely to occur once domain-group membership for an account exceeds 19.
Customers with SSO configured with multiple domain-based identity sources along with vCenter Server domain user accounts that are associated with a large number of groups should not upgrade to vCenter Server 5.1 Update 1.
--> Please see more on the VMware KBTV Blog : http://blogs.vmware.com/kb/2013/04/alert-login-issue-after-updating-to-vcenter-5-1-update-1.html