These two items strongly suggest that your origin is not configured correctly and lacks the required HTTPS. Flexible does not use HTTPS and is completely insecure. Your origin site needs to work over HTTPS before you introduce Cloudflare.
You need to pause Cloudflare and fix your origin. You can use commercial certificates or free automated certificates like those issued by the Let’s Encrypt CA. If neither of those are to your liking, you can use a free certificate from the Cloudflare Origin CA. It will display an unknown issuer warning to direct connections. It is trusted by the Cloudflare proxy and will work with Full (Strict), which is the only mode that is sufficiently secure.
I’ll escalate this post for the attention of the Customer Support Team so they can get back to you here. If you have already contacted Support, please share your ticket number here so that they can track it.
We have created ticket #2859453 in your Cloudflare account regarding this issue, and we will continue our investigation through the ticket. Please respond to the ticket when you have the chance. @orpobild