Yesterday upon trying to upgrade VirtualCenter 2.5 from U4 to U6, the result was that the VirtualCenter Server service refused to start, showing the famous error 2. I added the SQL Express dependency but this did not work, and I had to roll back to the previous night's backup. I did not receive any error messages during the process. Should I do U4 to U5, then U5 to U6?? So far as I could tell I followed the instructions correctly, I chose to keep the existing database. Thank you, Tom I'm running esx3.5
I recently had to upgrade our VC2.5 server as well -- unfortunately i came across several issues and landed up having to rebuild the entire system, reinstalling everything from scratch.
Turned out to be a giant pain in the ass, i lost all my resource pools... :(
If i were you i'd do U4 to U5, then to U6, and hopefully that will work out.
On a side note -- We run a system of about 50-60 Virtuals powered on 3 ESX 3.5 machines, and we removed the SQL Express component and installed MsSQL 2005 full. VirtualCenter 2.5 showed a night-and-day performance difference, Everything is significantly faster now and all previous complaints were nullified.