A month ago I had to upgrade one of our customers vCenter from 5.0 to 5.5. We were suppose to run pilot project and upgrade was part of the requirements. However, upgrade was pretty smooth with no unexpected issues or whatsoever. Of course, when I had all components installed I decided to do quick check, though. Login with C# client did not indicate any unusual behavior or possible error, but when open Web Client I saw error message that Web Client could not connect vCenter.