The local storage server cannot be marked as a CDN
Admin tried to mark the built-in local storage server as CDN (400).
Why this happens
Local and CDN are different backend types. CDN needs its own server row and plugin/credentials.
Diagnose and fix
1. Add a CDN server (providers overview) instead of converting local.
2. Configure CDN credentials separately.
3. Leave local as the primary local disk backend.
4. See CDN storage requires the CDN premium plugin.