ERR_SSL_PROTOCOL_ERROR Fix Guide

Problem

Users see ERR_SSL_PROTOCOL_ERROR, or HTTPS connections fail before the page loads.

Symptoms

• Browsers report an SSL or TLS protocol error.
• SSL checks cannot retrieve certificate information at all.
• HTTP is reachable, but HTTPS fails.

Top 3 Causes

1. Port 443 or the TLS service is not responding - Firewalls, listeners, or proxies prevent the handshake from starting.
2. TLS configuration is broken - Chain files, protocol settings, SNI, or termination config are wrong.
3. Redirect or proxy policy is inconsistent - HTTP-to-HTTPS behavior does not match the actual proxy/origin setup.

Diagnose with DechoNet

• SSL Check to see whether the handshake succeeds and certificate details are available.
• Port Check to verify port 443 is open externally.
• HTTP Check to inspect redirect behavior from HTTP to HTTPS.

Resolution Checklist

• Verify that port 443 is actually reachable from outside the network.
• Confirm the certificate, private key, and full chain deployment are correct.
• If a CDN or reverse proxy terminates TLS, make sure its HTTPS policy matches the origin.
• Check that HTTP-to-HTTPS redirects are not duplicated or conflicting.
• Re-run SSL Check and HTTP Check after changes to confirm the handshake and final response work normally.

When to Escalate

• Escalate to the hosting or CDN provider if you cannot access TLS termination settings directly.
• Escalate internally if multiple load balancer or proxy layers make certificate deployment ownership unclear.