Hi,
I see several posts where people are having difficulty logging into vCenter after upgrading. Before doing any upgrade do yourself a favor and take SQL and VM backups using something like Veeam. I have carried out a number of upgrades to 5.1a and then an in-place upgrade to 5.1b without any issues. This weekend I upgraded a customer from 5.0 to 5.1b as it is now recommended.
Just to add, I always remove the default groups under permissions in vCenter and add a vcenter admins group to tighten up security.
After the upgrade I couldn't login using the VI Client and got a permissions error, so I then went to the Web Client and could login using domain credentials but nothing appeared. It looked as if my DB was wiped.
I logged a call this morning and after 2.5 hours support took logs for review and I had another go at it. I managed to login to vCenter using the local Server admin account (servername\administrator) and hey presto I was in. I went straight to permissions and saw that my vCenter Admins group was there along with Administrator, but the domain part was missing. I added in domain\vcenter admins and all worked.
Just thought I'd post this as it may help someone.
