Are you sure your DNS and/or hosts file has not been hijacked?
For hosts you can check C:\Windows\System32\drivers\etc - open the hosts file in notepad.
It should look something like
this. If you have anything under the localhost lines, then it may have been modified by a virus.
Checking if your DNS has been hijacked is harder, but I'd highly recommend you run a full system scan using both avast and
MalwareBytes.
After doing all of the above, open Avast, click Additional Protection then SafeZone and click the Reset SafeZone button.
Either logout of your Windows account, and log back in again - or restart your computer. Then open SafeZone and see if you still have the same problem.
If resetting the SafeZone still does not fix the issue, then I would highly recommend repairing avast (using the option on the program uninstaller) and resetting SafeZone again to see if that makes any difference.