Thank you; I had missed that thread.
It seems that there is a problem upgrading with the AV fully in place and running normally. You posted in May so Avast have had at least 2 months to identify the cause of the problem. It also seems that some systems do not have the problem; Spec8472 reported that his ran the upgrade properly even with shields in place.
I recognise that upgrading Windows is a challenging activity but I would very much prefer to be able to do it without arguing with my virus scanner. It would be helpful if Avast responded to the problem, if not offering a solution now,at least in time for the 1909 upgrade. If not a solution, a warning would be helpful (and this may require reaching out to Microsoft).