After experiencing an almost identical problem, I can offer the following method to delete remaining corrupted Avast files after uninstalling...
Avast do have a little publicised 'Uninstall Utility' program -
https://www.avast.com/en-gb/uninstall-utility , which in my case after following the instructions, successfully deleted all remaining Avast files, so a 'clean' re-install could be implemented, thus finally reinstating the 'Adblock' and 'Bank Mode' extensions for my Avast Browser!