Have you checked in the log file "vmware-vci-log4cpp.log" It may shed light on what is really causing the "metadata missing" errors. On my system it is in this location:
"C:\Documents and Settings\All Users\Application Data\VMware\VMware Update Manager\Logs\vmware-vci-log4cpp.log"
The server that I have the Virtual Center Server software on is dual-nic'd. One address is routable, but the other is on a private, non-routed VLAN used for storage for certain ESX hosts.
By looking in it I found that my update scans were failing because the Update Manager was insisting on pointing to the non-routable address used by the second NIC in the server. The host I was trying to scan can't reach that address (it's at a different physical location).
By disabling the 2nd NIC I was able to verify that update scans would then work.