Windows Defender Setup Checklist for Small Business - Troubleshooting Cloudflare Error 521-Firefox Not Responding While Other Browsers Work: Fix
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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