There are already several reports of this type of behaviour (search the forum).

You could try 2 possibilities (either one of them or both).

1:
Avast main GUI → settings → troubleshooting → “load avast service only after other system services”.

2:
Avast main GUI → webshield → EXPERT settings → “…well known browsers only”.

In each case, please REBOOT after changing any of those settings before testing.