Try deleting avast processes from the applications lists in your firewalls, reboot your systems and wait for avast to be rediscovered then make sure that all avast processes are set to allow all, there should be three sevices - AvastSvc.exe, AvastUI.exe and Avast setup.

If that still has no effect try realtime shield\web shield\expert settings and tick the box to scan well known browser processes only.