ERR_EMPTY_RESPONSE: The Server Sent Nothing

• PHP-FPM/Apache worker crash or segfault
• nginx upstream timeout — backend did not answer in time
• OOM killer killed the PHP/worker
• DB connection pool exhausted — request waits and server drops connection
• Backend infinitely returns 499 (client closed) in a redirect loop
• Cloudflare Error 520 (server returns empty reply)

1. Check tail -f /var/log/nginx/error.log — look for "upstream prematurely closed" or "recv() failed"
2. PHP-FPM logs: tail -f /var/log/php8.x-fpm.log — look for "child exited on signal" or OOM
3. Raise memory_limit: php_admin_value[memory_limit] = 512M
4. Check status: systemctl status php-fpm nginx — all running?
5. Monitor uptime via Enterno Monitor — instant flap alerts
6. On Cloudflare — test origin: curl -H "Host: example.com" https://IP/

Check SSL Certificate →

server {
    listen 443 ssl http2;
    server_name example.com;

    ssl_certificate     /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

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

    ssl_stapling        on;
    ssl_stapling_verify on;
}

• NET::ERR_CERT_AUTHORITY_INVALID
• ERR_CERT_DATE_INVALID
• ERR_SSL_PROTOCOL_ERROR
• SSL Handshake Failed
• All SSL errors