no https

I run WP with the CommentPress plugin. I have a free “Let’s Encrypt SSL” certificate installed. Yet, any attempt at connecting to my WP site via https reverts to http. The apparent culprit is “mixed content”. I have examined user accessible templates, style sheets and images. None contains an explicit http call. One of the test plugins claimed it was due to a “reverse proxy”, so I threw out DreamFlare.
In Chrome, I get : “NET::ERR_CERT_COMMON_NAME_INVALID”.
Is the problem in the server, the proxy, the SSL certificate, WordPress, CommentPress, in the content, or somewhere else?