I had this problem today also for the first time. I tried all sort of things from restoring licences to rebuilding databases but to no avail. Then I remembered resetting my network settings to auto the other day. I usually use the google DNS settings (8.8.8.8 & 8.8.4.4) for faster downloads. I switched my DNS back to the google ones and the update working straight away. Could be worth a try.

I am no technical expert and have no idea if this could, would or should do anything at all, or if it was just a coincidence. Good Luck.