VMware Update Manager = Windows Server 2008 R2
vCenter = vCenter Server Appliance
Over the last couple of days I've been investigating an issue where VMware Update Manager tasks - "Check new notifications" and "Download patch definitions" complete OK when manually run, but sit in the task pane as "Queued" when run through the scheduled task.
Hunting around online for a solution only provided me with a way to clear the tasks, not to actually fix the issue (for reference, this is to restart the VMware vSphere Update Manager Service on the vUM server).
After investigation, the queuing issue appears to be due to permissions problems. The services on vUM were running as Local System account. I changed this to an account with permissions in vCenter - in this case the Update Manager domain account I had created - and re-ran the scheduled task. The task now completed successfully each time.
After changing the account remember to restart the service for the change to take effect.