In Update Manager of vSphere Client 5.1, when you are attempting to ‘remediate entity’ to apply some patches, you get this error:

“VMware vSphere Update Manager had an unknown error.  Check the events and log files for details.”

This may be because there is an ISO mounted in the CD/DVD drive of one of the VMs that will need to be vMotion’d.  Removing the media by switching over to ‘Client Device’ in Edit Settings may resolve it.