I got the same problem when upgrading from Avast free to Avast Pro. Still looking for a solution, I'll post it here if I find something. I was definitely not expecting this...
The solution is removing the Avast Online Security add-on.
You can remove/disable it... the web shield deals with all the HTTP traffic/malware on the web.
To remove completely : Control Panel > Program and Features > Uninstall a Program > Avast > Change > [uncheck] Browser Protection
Note : You may need to reboot or restart your browser for the change to take effect.