The validation request to this domain was blocked by a deployment of the nginx testcookie module (https://github.com/kyprizel/testcookie-nginx-module). This module is designed to block robots, and causes the Let's Encrypt validation process to fail. The server administrator can solve this issue by disabling the module (`testcookie off;`) for requests under the path of `/.well-known/acme-challenge/`.
The server at 185.27.134.135 produced this result.