How can I diagnose a 500 error in Nginx when the error logs are silent?

Start by checking the logs of your upstream application (PHP-FPM, Node.js, Python WSGI server, etc.). Then, examine system logs (e.g., `/var/log/syslog` or `/var/log/messages`) for out-of-memory (OOM) killer events or other system-level errors. Use tools like `top` or `htop` to monitor resource usage. Try temporarily increasing logging levels in your upstream application for more detailed debugging information. Also, check file permissions on all files accessed by the upstream application.