The HTTP Error Response Code 526 occurs when Cloudflare is unable to successfully validate the SSL certificate on the origin web server and the Cloudflare SSL configuration on the website is set to "Full SSL (Strict)".
Server administrators should review their SSL certificates to ensure that they meet all of the following validation requirements:
- The certificate expiration date has not already passed
- The certificate is signed by a certificate authority (such as GlobalSign, Verisign, GeoTrust, Comodo, etc) and is not a self-signed SSL certificate.
- The requested domain name (hostname) is in the Common Name or Subject Alternative Name configuration of the certificate.
If the intent is to have Cloudflare connect to the origin using SSL and the origin is configured to use a self-signed certificate, the domain needs to be configured to use Full SSL and not Full SSL (Strict) in the Cloudflare settings.