Thanks for the steer Fen.
In Norton I found a place (under Security/Advanced/Antivirus/Exclusions) where I could exclude the folder where I keep AL and all my Thief FMs. AL seems to be working normally again now. It's odd how Norton did not report what it was doing (it isn't shy about putting up messages), and how the problem came out of nowhere recently, but it seems to be fixed now.