The website suddenly displays HTTP 521, typically caused by abnormal responses from the origin server, incorrect CDN configuration, or firewall blocking. This article outlines five common scenarios and troubleshooting steps to help restore normal access quickly.

Analyzing the Core Causes of Frequent HTTP 521 Errors During Peak Traffic Periods: A Comprehensive Examination from Concurrency Limits and Server Performance to Network Bottlenecks, Along with Actionable Optimization Solutions to Enhance Website Stability.