From ZDenek (avast team):

Hallo, we have discovered an issue in new WebShield implementation. This is probably reason of your problems. We are preparing fix for that.

Meanwhile you can try to switch to old WebShield implementation. To do that, please
Go to C:\programdata\avast software\avast directory
Find the WebShield.ini, file and add following lines:

[General]
UseStreamFilter=0

Now restart „avast! Antivirus“ service, or reboot.


That could solve it.

ps: You may already have a hardware firewall in your router/modem. If you have, there is no need to have a software one as well.