Hi,
Did you check this KB :http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1017377
What is your ESXi and vCenter Version. Can you attached the vum log files please.
"(located in C:\Documents and Settings\All Users\Application Data\VMware\Update Manager or C:\Users\All Users\VMware\VMware Update Manager\Logs): "
Regards,
julien.