526 Invalid SSL Certificate
Meaning
A Cloudflare-specific status indicating that the origin server presented an SSL/TLS certificate that Cloudflare considered invalid (e.g., self-signed, expired, or otherwise untrusted).
When to Use
When the proxy can connect over TLS but the certificate validation fails. Cloudflare uses this to signal to the user that the handshake was made but the certificate from the origin is not acceptable (no trusted CA, CN mismatch, etc.).
Example
If a website’s origin has an expired SSL certificate, Cloudflare may return 526 Invalid SSL Certificate to the end-user, essentially saying it cannot establish a trusted secure connection to the origin due to certificate issues.
When Not to Use
Do not use on a typical server directly to clients. A normal client connecting to a server with an invalid cert will not receive an HTTP 526; instead, the TLS connection would fail entirely. 526 is a byproduct of Cloudflare’s attempt to connect on behalf of the user.
Source
Unofficial (Cloudflare)