I would restart your vCenter Server Service along with your Update Manager Service.
I've seen these get "stuck" in the DB too. If Troy's suggestion doesn't work, get your DBA to check the DB for errant entries.
I can confirm the reboot of the vServer will get rid of the Status. I had the same issue.
On other discussion some people say that is goes in timeout in a couple of hours.
But the best solution is the one above (from Troy) to restart your vCenter Server Service along with your Update Manager Service.
Sometime the VUM restart hang up, in this case you can simple restart the entire VC server.
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Ten Points to the Giant. I would like to add that when VUM hangs during the restart you can always end the process via taskmanager or use kill if you have it installed.
What I did...
I've just restarted the service mgmt-vmware on the affected server with the following command: service mgmt-vmware restart
and after I restarted vCenter service on windows machine:
net stop vpxd
net start vpxd
These steps above for me worked fine.
Try removing guest from inventory. Restart vCenter server, then re-add guest back to inventory