Came here to report the same issue, and found this thread already open. The problem started in version 6.8.25. Version 6.8.24 did not have this problem, I made several installations. I tested it on CentOS 7 and Ubuntu 20.04, and on both there is this problem when enabling SSL from the Panel when version 6.8.25 was released.
This is exactly what happens: after linking a domain to the Panel and enabling SSL with Let's Encrypt, the panel informs that SSL has been enabled and soon after the page is refreshed, the browser displays the error "ERR_SSL_PROTOCOL_ERROR".
I checked in the directory /www/server/panel/vhost/letsencrypt if the Let's Encrypt certificate was generated, and it is there, it was generated, but the directory /www/server/panel/ssl was not created with the files of certificates used by the Panel, so after creating it manually and creating the files "certificate.pem" and "privateKey.pem", the panel loaded again with SSL enabled.
