There are some special things you need to pay attention to when using Cloudflare with DiscourseHosting.
In general, just use the most default setup. Don’t use any advanced caching rules and stuff like that. If you must, because you have other websites in your domain that require this, then just make sure your Discourse forum bypasses the Cloudflare servers by going to the DNS menu and pressing the orange cloud next to your Discourse hostname so it turns to grey.
Alternatively, read on.
Set SSL/TLS – Encryption Mode to “full”
Don’t set it to “Off” and definitely don’t set it to “Flexible” either.
Don’t enable any of the settings in the Speed menu.
Just stay away from them. If you have other sites in your domain that require this, bypass Cloudflare for your forum as explained in the first paragraph of this article.
Do not redirect http to https using a page rule.
We do this for you, and if you do it using Cloudflare, we cannot issue SSL certificates for your domain anymore, and our servers will run into issues.