bbmmzzz: Thanks for your reply. Your suggestion worked.

For this rare fault it is an easier solution than updating the whole program across 3 pcs. Last time I updated the Avast free antivirus was when an earlier version refused to accept a new licence key after the first year. I only use the file shield so the other features that have been added over the years are lost on me. However reliable operation is important, and apart from the licence key issue and now this online update problem, I have found Avast works and I have been using it since at least 2010.