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.
1. Origin Server Is Offline
The most straightforward cause is that the web server is completely down.
This can happen due to:
Server crash
Power outage at the data center
Hosting provider downtime
Accidental server shutdown
Kernel panic or OS failure
Operating Systems
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