How to Troubleshoot and Fix Common HTTP Error Codes

Where to find the logs

sudo tail -f /var/log/nginx/error.log
sudo tail -f /var/log/apache2/error.log
sudo journalctl -u php8.3-fpm --since "10 min ago"

403 Forbidden

• File permissions: directories usually 755, files 644, owned by the web user.
• No index file and directory listing disabled.
• A deny rule in Nginx config or .htaccess, or a security plugin/WAF blocking your IP.

sudo find /var/www/example.ca -type d -exec chmod 755 {} \;
sudo find /var/www/example.ca -type f -exec chmod 644 {} \;

404 Not Found

The URL does not match a file or route. Check try_files in Nginx, rewrite rules in .htaccess, and CMS permalink settings. For moved content, add a 301 redirect rather than leaving a 404 — and design a helpful custom 404 page.

500 Internal Server Error

• PHP fatal error: look for 'PHP Fatal error' in the error log.
• Syntax error in .htaccess on Apache.
• Exhausted memory_limit or a failing plugin after an update.

502 Bad Gateway

Nginx could not get a valid response from the upstream (PHP-FPM, Node.js, a proxy). Typical log line: 'connect() to unix:/run/php/php8.3-fpm.sock failed'. Confirm the service is running and the socket path matches.

systemctl status php8.3-fpm
ls -l /run/php/

504 Gateway Timeout

The upstream accepted the request but took too long. Find the slow request (slow query log, PHP-FPM slowlog), fix the cause, and only then consider raising fastcgi_read_timeout. If all PHP-FPM workers are busy, revisit pm.max_children in our LEMP guide.

More on performance in web development and server setup in server infrastructure.