How was the update task configured for the reboot, was this a "Reboot if required.." option, or an "Always reboot" option?
Patch Manager actually launches that reboot via a WMI call after noting the completion of all update installation tasks. A relevant question would be whether the server was rebooted at all, or not. It's possible that the server was already in a reboot sequence and the attempt to send the Reboot command via WMI is what failed. Note, also, in this case, that the Task Results really only record that the request was sent. Patch Manager does not actually monitor whether the reboot occurs or not -- in the Post-Updates scenario. (It does monitor the reboot in a Pre-Update reboot scenario.) So this failure is not an indication that the reboot failed only that the transmission of the request to reboot failed.
Does the Task History | Details tab provide any additional information on the reason for the task failed status?