I will repeat what I said above that no Plesk developers have been known to participate in this forum and that we can’t easily debug Plesk-related problems. Generally only developers of the open-source Let’s Encrypt clients have personally participated in this forum. There is a Plesk-specific forum at
which would probably be more helpful.
To add something from another thread, most of the recent mysterious challenge failures (whether with Plesk or other clients) have been associated with Let’s Encrypt’s switch to prefer validations over IPv6 instead of IPv4. We’ve seen that many users have an IPv6 address that they’re advertising in DNS with an AAAA record, which they may not even have known or remembered about. Let’s Encrypt will try to connect to that address over IPv6 to perform the validation, and if there’s a timeout or 404 error, the challenge will be marked as failed. There is no direct evidence that that’s the specific reason for either of your problems (and redacting the domain name has prevented me from checking for myself!) but this is a good thing to look into in general. I would say that this has accounted for up to 60% of the problems that people have encountered with failed HTTP-01 challenges on all platforms since the change took effect.