So the virus update definition update worked from Windows Update today.
At least that tells us WU is working OK.
I wondered if something with WU broke with the last patch Tuesday updates for you.
I can't explain and have no idea why the scan starting work again after using WU today.
You should NOT have to use WU to update the Virus definitions to get the scan to work as scheduled.
I don't use WU for definition updates, i set up a scheduled task to run every 4 hours to check for and install new definitions.
I can help you set that up if you're interested.
But, it doesn't explain the Scan "issue", and i've never seen a problem using the built in MSE Scan scheduler.