When browsing a website, few things are more frustrating than encountering an error page instead of the content you were looking for. One such error that often confuses website owners and visitors alike is “Web Server Is Down – Error Code 521.” This error is most commonly associated with Cloudflare, a popular content delivery network (CDN) and security service.
If you’ve ever seen Error 521 on your site—or while trying to access someone else’s—you may have wondered what it actually means, why it happens, and how to fix it. This article provides a complete, in-depth explanation of Error 521, including its causes, troubleshooting steps, and best practices to prevent it from happening again.
The origin server responds.
Computer Servers
Cloudflare delivers the content to the visitor.
Error 521 occurs at step 3 or 4—when Cloudflare tries to talk to your web server, but the server doesn’t respond properly.
Common Symptoms of Error 521
Error 521 can appear in several ways depending on your role:
For Visitors
Website does not load
Cloudflare error page appears
Message: “Web server is down”
For Website Owners
Sudden traffic drop
Users reporting downtime
Server logs show refused connections
Cloudflare dashboard shows 521 errors
Primary Causes of Error 521
There is no single cause of Error 521. Instead, it usually results from one or more server-side issues. Below are the most common causes explained in detail.
ADVERTISEMENT
