A Cloudflare 521, 525 or 526 error usually means something has gone wrong between Cloudflare and the website's origin server. The common mistake is changing Cloudflare's SSL/TLS mode before checking what the origin actually returns. That can hide one problem while creating another, especially when switching to Flexible mode.

Quick answer: Error 521 means the origin refused Cloudflare's connection. Error 525 means the TLS handshake between Cloudflare and the origin failed. Error 526 means Cloudflare couldn't validate the origin's certificate in Full (strict) mode.…