Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
I'm not sure exactly which "account owner" you are trying to find. The only account credentials you might want for Let's Encrypt are the ACME staging and production account private keys. If those are lost, new accounts can easily be generated with likely no real loss.
I'm guessing that for whatever you're trying to get access to, you need to contact the hosting provider or registrar. All Let's Encrypt does is confirm which computers own a name so that end-users can be assured their connections to that name are secure. It doesn't actually relate to the content or administration of a site, other than the administrators of a site might use Let's Encrypt's services as part of their setup, if that makes sense.
Let's Encrypt doesn't run any nameserver hosting services to my knowledge. The server you have mentioned does serve a Let's Encrypt certificate via HTTPS over port 443. So do many millions of other servers including my own.
