I go the exact same error and symptoms when I upgraded from vCenterAppliance 5.1a to 5.1 Update1.
Fixed it by adding the entry 'lsass' to the /etc/nsswitch.conf file in the vCA, then restarting vpxd service. KB is here:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=205070...
Cheers,
Dan.
