Hi anon123,
To allow a server exception for Mac Software Updates and the App Store without disabling the avast! Web Shield you need to do the following:
Open your avast! Preferences,
Click the Lock in the left bottom corner,
Enter your Mac Administrator Password,
Click on Web Shield Advanced button,
Click Exclusions,
Click the + button in Lower Left Corner,
Click on the Form Field and type in swscan.apple.com,
Select Service as https,
Click Done,
You will then need to add a second Exclusion to the avast! Web shield,
Using the steps detailed above,
Add swdist.apple.com,
Select Service as https,
Click Done,
Then select the General Tab in the avast Web Shield Preferences,
Unselect Scan Secured Connections,
Click Done,
Click the Padlock in the Lower Left hand Corner to lock your avast! Preferences.
You will now be able to get all your essential Mac updates.
I personal think that is quite shameful that a basic avast! configuration error like this has existed since avast! for Mac was first made available.
It simply would NOT take a great deal of time or effort for avast! to get this fixed, But yet this problem still remains.