Explains the meaning and fixes of 502 Proxy Error error, covers troubleshooting steps for Nginx, Apache, Cloudflare and other scenarios, and helps you to quickly solve server gateway error problems.

What does error code 521 mean? This article delves into the true meaning of 521 and summarizes the three most common server configuration pitfalls beginners overlook—including firewall blocking, web service anomalies, and port listening issues—to help you quickly pinpoint and resolve the 521 error.