I have been pursuing this problem for some weeks now (I am not very PC literate - I am 81) - but what fascinates me in all this is that the onus seems all the time to be on the user - Uninstall; Reinstall; use Avastclear; Safe mode; Online Installer; Offline Installer ; Try this; try that etc. etc. All very confusing - but where is Avast in all this. It was working on my PC before update 2291. My system has not changed - so presumably something in the update caused the problem.
Perhaps Avast subscribes to Salvador Dali's dictum
“What is important is to spread confusion, not eliminate it.”