521: Web server is down

Likely cause

Cloudflare could not connect to your origin server at all — the server process is down or not listening on the expected port.

Check

Confirm your origin web server process is running and listening on the port Cloudflare is configured to use.

Fix

Restart the origin web server process.

sudo systemctl restart nginx
sudo systemctl status nginx

Verify

Retry the request through Cloudflare and confirm the site loads.

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