This will probably be a support ticket, but I'll throw this one out there...
After upgrading to 5.1 (all seemed to go fine), after every 3-4 days, I'm always having to reboot the vCenter server b/c I can't login. From the vsphere client, it simply says 'a general error has occured: Authorize Exception'. If I restart the vCenter service, that doesn't help - I have to reboot the whole server, and then I can login fine.
This vCenter server was upgraded from 5.0, and previously from 4.1. The event logs are completely clean, and all the services are running. AD is clean and happy and everything is good as far as I can tell.