Have a look at the logfiles
- Location of vCenter Server log files (1021804)
- Location of vCenter Inventory Service log files (2056632)
- Location of vCenter Single Sign-On log files for vCenter Server 5.1 and 5.5 (2033430)
Probably one of the components (SSO/vCenter/IS) is likely to cause a problem, hopefully you can find more information in the logfiles to see where it goes wrong