I experienced the exact same problem.
In the end I solved these problems by uninstalling VMware vCenter Update Manager, using Add or Remove Programs from the Control Panel, and installing it again with a clean database but if its not causing you immediate pain maybe a closer examination of the logs will help (I was in a rush at the time) - can find the logs at C:\Documents and Settings\All Users\Application Data\VMware\VMware Update Manager\Logs - upload a copy if you want and we run through it.
The log directory was helpful. After finding it I did a search for the errors I was seeing. I came across the post below.
I shutdown the vma and then the patch definitions dowloaded.