526 Invalid SSL certificate
HTTP response status code 526 Invalid SSL certificate is an unofficial server error that is specific to Cloudflare. This HTTP status code occurs when a HTTP Connection cannot be made to the origin server due to an invalid SSL certificate.
Usage
Cloudflare is a content delivery network that acts as a gateway between a user and a website server. When the 526 Invalid SSL certificate status code is received, Cloudflare is unable to validate the SSL certificate on the origin server, and the SSL settings in Cloudflare SSL/TLS app is set to full SSL (Strict) for the website. The most common reason for this error to occur is that the SSL certificate on the origin server may be expired.
Takeaway
The 526 Invalid SSL certificate status code is a Cloudflare-specific server error that is generated because Cloudflare is unable to validate the SSL certificate on the origin server, and thus unable to create a secure HTTP Connection.