I created a USB rescue disk last night through the Avast Free Antivirus interface by clicking on Create on USB. Downloading the files took forever on my extremely slow mobile connection but eventually showed success. However, when I tried to boot up an infected computer using the disk, the program barely started and said "Unable to launch main Avast Rescue Disk Shell, aswShell.exe!". I tried using the same USB drive on another computer with the same result, then used another USB drive and still with the same result. Browsing through the forums, I see that somebody else had already reported the same problem on April 12 but writing in French on an English-language forum (
https://forum.avast.com/index.php?topic=189688.msg1542516#msg1542516), so his message might have gone unnoticed.
I'm using Avast Free Antivirus 20.2.2401, version 20.2.5130.570, on 64-bit Windows 8.1, with the latest virus definitions (200418-0). My user interface and system language is French if it makes any difference. The rescue disk's error message was in English in any case.