for my domain 9to5jailbreak.com I redirect from 1and1 where I bought it to Cloudflare and then to builderall. This setup ran for months and it does for a different domain too.
If I turn proxy on (which I want) then I get this error:
The host is where your site is hosted. And the server certificate is the certificate on your host.
You best pause Cloudflare for now (Overview screen, bottom right) and fix your server and make sure it loads fine on HTTPS and unpause only when you have fixed that.
Right, if your server IP address ends in 115, then you currently do not have a valid certificate on your server. You need to talk to your host about that and they need to address that.
And no, you are not using SSL from Cloudflare. You still need a valid certificate on your server.