Common Causes of ERR_SSL_PROTOCOL_ERROR
- Server listening on plain HTTP on port 443: Web server responding with unencrypted HTTP data to TLS client hello.
- QUIC / HTTP/3 UDP Handshake Drop: Firewall dropping UDP port 443 packets while advertising HTTP/3 in Alt-Svc headers.
- Corrupt SSL Cache in Browser: Stale TLS session tickets stored in client memory.
- Firewall / Deep Packet Inspection (DPI): Corporate firewall terminating TLS connections abruptly.
Diagnostic Commands
Use OpenSSL to inspect the exact handshake stage failure:
openssl s_client -connect example.com:443 -tls1_3 -debug⚡ Test Live TLS Handshake Health
Verify if your server completes standard TLS handshakes without errors.
Run Free Handshake Test →