Glad you were able to confirm.
Error, by itself, does not compromise the way avast provides protection, which is why I'd never done a scenario by scenario test for this.
Details matter, and not surprised this one was missed as long as it has been.
I put this down to perspective. To all, or most of the users complaining of this bug, they would (mostly) minimize the avast gui and continue to work in the same window. When it is finished, there would be the error. The difference in perspective is due to the fact most, if not all, believe a manual scan will catch something when a real-time scan will not.
Which is why they do it.
But I would not minimize, I would move to an empty desktop, and allow the scan to complete and so I would never see the error.
Just to be clear, a manual scan does not catch anything 99% of the time. Just about the only thing a manual scan is good for, imo.
If it does, it is an archived file which cannot do harm until it is opened and run.