I have vCenter Mobile Access running on ESX 4. When the VMware Update Manager attempts to download patch definitions, it generates the error message "VMware vCenter Update Manager has an unknown failure. Check Tasks and Events tab and logs for more details." Checking Tasks and Events, there are two related errors: "Repository address not set for vitual appliance vCenter Mobile Access, it doesn't support updates by VC" and "Failed to scan vCenter Mobile Access for VA upgrades".

Shutting down the vCenter Mobile Access appliance makes this error go away.

The release notes for VMware Update Manager 4.0 b162871 (http://www.vmware.com/support/vsphere4/doc/vsp_vum_40_rel_notes.html) describe a known issue that may relate to this:

VMware Tools Upgrade Remediation Might Fail with a Misleading Event Error Message
When you try to upgrade VMware Tools on a virtual machine with no installed VMware Tools, the remediation fails with a misleading error message:<code>VMware vCenter Update Manager had an unknown failure. Check Tasks and Events tab and logs for more details.</code>Update Manager does not support VMware Tools upgrade for virtual machines on which VMware Tools is not installed.

Busy as you no doubt are, if you get the chance to rebuild the vCenter Mobile Access appliance with the current ESX 4 VMware Tools, it would be interesting to see if that makes this error go away.

Thanks. Jeff.