After yesterday's problems I ran Ad Aware and Malwarebytes which found 4 'things' they didn't like, cleared them out, rebooted, and Avast is now back running again. The number of days left on my subscription is correct. I don't think that my Avast could have updated overnight (could it?, because it appeared to be out of subscription at that point) so it looks like there may have been a problem as described.