I have the same problem: Avast will show an annoying popup whenever I am connected to an unsecured wifi network, even though I clicked the checkbox that says “don’t display this message again”. (By the way, I am using the same wifi network all the time.) It seems to me that the problem is the checkbox itself: I can click on it, but it is never actually checked. Instead, a click on the checkbox immediately closes the popup.

I haven’t found a way to disable this popup via the avast preferences.