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How to Fix Cloudflare 521 Error During Setup - How to Reset Windows Defender Real-Time Protection-Fix DNS Probe Finished NXDOMAIN on Custom DNS Server

Updated 2026-07-28 15:15:34developer-tools
How to Fix Cloudflare 521 Error During Setup - How to Reset Windows Defender Real-Time Protection-Fix DNS Probe Finished NXDOMAIN on Custom DNS Server
Likely symptom
Likely cause
Safe rollback

Verification route

  • Repeat the original action, not only a simplified test case.
  • Restart the app or reload the service after configuration changes.

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