Hi, thanks for getting in touch. There’s a known bug that affects some users related to the Let’s Encrypt account id. Can you try setting your account email again under Settings (you can update it to the same email address), this will reset the Let’s Encrypt account id internally.
By default the app will use the builtin http challenge server, so if that works OK then none of the website directory.web.config setting matter, plus the error message your seeing is that the response doesn’t quite match what’s expected not that it doesn’t get the response at all.