Hello, everyone. Lisha is on as an IT administrator. Many WordPress users are perplexed as to why the cloudflare error 521 message arrives. Cloudflare gets error 521 when becomes unable to create a Tcp session to your source server or failed to respond to your origin server via port 80 or 443. Security or firewall software is by far the most common cause of error 521. When the origin server expressly declines Cloudflare's proxy request, this happens.