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
If the server is offline, Cloudflare has nothing to connect to, resulting in Error 521.
2. Web Server Is Overloaded
When a server runs out of resources, it may stop accepting new connections.
Common overload causes include:
Traffic spikes
DDoS attacks
Poorly optimized applications
Memory leaks
Insufficient CPU or RAM
When the server reaches its connection limit, it may actively refuse Cloudflare’s requests, triggering Error 521.
3. Firewall Blocking Cloudflare IP Addresses
One of the most common causes of Error 521 is a misconfigured firewall.
Computer Servers
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