I'm getting exactly the same via Plesk -
Error: Let's Encrypt SSL certificate installation failed: Could not obtain directory: Invalid response: <HTML><HEAD><TITLE>Error</TITLE></HEAD><BODY>
An error occurred while processing your request.<p>
Using Plesk 12.5.30 Update #66
Let's Encrypt plug in version: 2.1.0 Release: 48
Hope that helps get a resolution.....