Did you try my solution to the missing file (see the bottom on my post above)? That’s what I had to do to get it to work.
I even seem to recall an error message along the same lines as you say.
- go to your Avast installation directory
- go to the “Setup” subdirectory
- from Explorer, rt click New->Text Document. Call it “avast.setup”
- set permissions for this file in PG (as shown above)
- delete the avast.setup file.
Good luck