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.