ERR_SSL_PROTOCOL_ERROR Troubleshooting

Protocol errors occur when servers and browsers cannot agree on encryption methods. This often results from outdated server configurations supporting only obsolete protocols. Modern browsers require TLS 1.2 minimum, with TLS 1.3 preferred. Check your server's protocol support using:

# Test SSL/TLS protocols
nmap --script ssl-enum-ciphers -p 443 example.com

# Or using OpenSSL
openssl s_client -connect example.com:443 -tls1_2

Update server configurations to support modern protocols. For Nginx:

ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384;
ssl_prefer_server_ciphers off;