I’m assuming you are using the new Avast 9 beta?
If so, the only way I’ve found to get Apple Updates (and a few other services) to work is to disable “scan secured connections”.
Do this through the preferences - web shield - advanced - general
This was a problem in the betas of version 8 as well, I really don’t understand why Avast has not fixed it, or at least set the default setting to disabled.