Issue
You get a 526 error response from Cloudflare, even though Cloudflare has a valid SSL certificate for your site.
Resolution
Go to the sites Cloudfare dashboard, SSL/TSL Overview tab and change the "SSL/TSL encryption mode" from "Full (strict)" to "Full".
Cause
Cloudflare was requiring the cert at the Acquia Lopud balancer to have your domain name and be valid, but the valid `*.acquia-sites.com` cert was not sufficient.