Greetings,
Are you using Cloudflare Load Balancing feature or rather doing “load balancing” on your origin host/server via HAProxy or Nginx or some other? 
May I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?
There are topics about this and this is an ongoing warning on the dashboard which is going to be resolved soon I believe: