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Fix Docker Compose Environment Variable Default Value Not Used - How to Resolve Error 521 Web Server Down-Troubleshooting DNS Probe NXDOMAIN VPN Issue

Updated 2026-07-28 10:09:20office-tools

Fix Docker Compose Environment Variable Default Value Not Used - How to Resolve Error 521 Web Server Down-Troubleshooting DNS Probe NXDOMAIN VPN Issue

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.