A comprehensive diagnosis manual for identifying and resolving browser security warnings, TLS handshake failures, and intermediate certificate chain breaks.
When diagnosing an SSL error, never replace the certificate blindly. First identify which of the three distinct connection stages failed:
The client attempted HTTPS on port 443, but the server returned plain HTTP or reset the connection before TLS negotiation took place.
Client and server could not negotiate a compatible TLS version (1.2/1.3) or cipher suite. Common in Cloudflare Error 525 origin issues.
The certificate was received, but rejected by the browser due to an incomplete chain, hostname mismatch, expiration, or untrusted issuer.
Origin server port 443 is unreachable or origin certificate is invalid while Cloudflare is set to Full (strict). Verify origin TLS listening.
The web server omitted the intermediate CA bundle. Download the CA bundle and link it in SSLCertificateChainFile or fullchain.pem.
The accessed URL does not match any SAN in the certificate. Reissue with proper multi-domain coverage or use a wildcard certificate.
Secure HTTPS page loads insecure http:// resources. Add Content-Security-Policy: upgrade-insecure-requests or update asset URLs.
The certificate expired or client clock is incorrect. Renew immediately via RenewalSSL control panel and reload web server.
HTTPS traffic sent to a non-TLS port. Configure server-level 301 redirection from HTTP port 80 to HTTPS port 443.
openssl s_client -connect example.com:443 -servername example.comopenssl x509 -noout -modulus -in cert.crt | openssl md5
openssl rsa -noout -modulus -in key.key | openssl md5Browse our comprehensive catalog of DV, OV, and EV certificates from Sectigo, DigiCert, and GeoTrust with instant issuance and 24/7 technical validation support.
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