I have just upgraded from Vcenter 5.0 to 5.1 and had an issue which was resolved but I thought I would share it just in case anyone else has the same problem. We had a single Vcenter 5.0 server and not many hosts so I did the simple install. The SSO and inventory service all completed without issue but when the Vcenter 5.1 install finished, it informed me that there was one error but didn't give any details (at least I could find any). I went to login via the vsphere client it gave me an error which went along the lines of 'an unknown connection error occurred could not connect to the remote server'. Tried the web client, similar message. I then checked Windows services, the SSO, Inventory and various other services had started but the Vcenter service had not and didn't start manually. Tried rebooting, no dice. I checked the vpxd log and it listed this:
[06436 error 'utilvpxdVdb'] [VpxdVdb::SetDBType]: Login failure - retrying once...
[06436 error 'utilvpxdVdb'] [VpxdVdb::SetDBType] Failed to connect to database: . Retry attempt: 1 ...
[06436 info 'utilvpxdVdb'] [VpxdVdb::SetDBType] Logging in to DSN: VMWare_VCenterDB with username
[06436 error 'utilvpxdVdb'] [VpxdVdb::SetDBType] Encountered login error. Subsequent connection attempt failed: 28000
[06436 error 'utilvpxdVdb'] [VpxdVdb::SetDBType] Aborting after 1 retries.
[06436 error 'Default'] Error getting configuration info from the database
Seeing the database mentioned I tried adding various users and permissions to the SSO database and Vcenter DB but still didn't work so I logged a call with VMware support. All they did was go to the Vcenter service and change the 'Log on as' account from the one defined to a local system account. That enabled me to login as normal, and if I recall correctly they did the same for the Webclient service as well.