Latest release of VMware vCenter 5.1U1a has been released yesterday. This latest release comes up after several weeks only, because a fix was discovered just after the release of 5.1U1. A bug which affected users which were logging into vCenter Server using the vSphere Client with vCenter Single Sign-On (SSO) in a multi-domain environment. Those login fails for users in multi domain environments. Users which are present in more than 19 security groups, as you could probably read this VMware KB article 2037410 which reports on that.
Check out the release notes of VMware vCenter 5.1U1a before start the upgrade from the 5.1U1 release.
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. Customers are urged to read the README included with the new update before they apply the update. Details of what has and has not been fixed are provided in KB article: https://kb.vmware.com/kb/2037410
The scenarios where the SSO problems were identified affected users in in multi domain environments where users were present in a lot of groups (19 or more in most parts!), so if this is not your case you're not affected.
The original alert from VMware:
Download here the vSphere 5.1 U1a from VMware website : VMware Download Center
The errors that were generated were:
The vSphere Client could not connect to “<vCenter Server name>“. You do not have permission to login to the server: <servername>
If you're running production environment, and want to jump into an upgrade process, the most important thing is to ….. wait! Wait at least two weeks after each release to see which bugs are discovered by users.