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How to Fix Cloudflare 521 Error on Apache Servers - DNS Probe Finished NXDOMAIN Fix Guide-Troubleshooting Chrome Sync Failure with VPN

Updated 2026-07-28 03:19:03macos-help
How to Fix Cloudflare 521 Error on Apache Servers - DNS Probe Finished NXDOMAIN Fix Guide-Troubleshooting Chrome Sync Failure with VPN
Before changing system settings, save your work and note the exact error text so you can compare results after each step.

Extra verification checklist

  • Restart the affected app after changing permissions or configuration files.
  • Test the same workflow in a clean browser profile or a temporary user account.
  • Compare the result on another network if the error may be connection related.

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