I tested, it loads fine at my end over HTTPS with the response from the content value of “TREST”.
Is that as it should be for now?
What SSL options have you got selected under SSL tab/settings page at Cloudflare dashboard for your domain?
- Full SSL, Flexible SSL, Always Use HTTPS, Automatic HTTPS redirection …
Moreover, have you got an SSL certificate installed at your host/origin which covers your main domain and sub-domain(s)?
Is the host/origin server working on HTTPS port compatible with Cloudflare?
Is the Cloudflare allowed to connect to your host/origin?