I doubt it, avast doesn’t do anything to your email other than scan it for infection. Depending on your OS (and you don’t mention which it is) and/or email program (again no mention) your account settings may need to be customised (but this doesen’t appear to be the problem, as you wouldn’t be able to send or receive email if it wasn’t set-up correctly).

I would email Yahoo Groups support asking why this has happened and also visit their FAQ/Support pages and see if there is/are any problems like this already mentioned (I doubt that this is the first time this has happened).