Security
SSL certificate error: identify name, chain or date problems
A browser certificate warning is intentionally disruptive because the browser cannot establish the expected authenticated TLS connection. The exact error tells you whether to investigate the hostname, validity period, trust chain, server configuration, or client time.
Check the requested hostname against the certificate names
A certificate for `example.com` does not automatically cover every unrelated hostname. Make sure the server is presenting a certificate whose subject alternative names include the hostname the visitor requested.
Check validity dates and system time
An expired certificate or one not yet valid can be rejected. Incorrect time on the server or client can also make a valid certificate appear outside its allowed period.
Verify the intermediate chain
Servers normally need to present the leaf certificate plus required intermediate certificates so clients can build a path to a trusted root. A missing intermediate can fail on some clients while appearing fine on others that cached it previously.
Make sure the correct virtual host serves the certificate
Reverse proxies and multi-domain servers can present the wrong certificate when SNI/virtual-host configuration is incomplete. Test the public hostname from outside the server rather than inspecting only the certificate files on disk.