Invalid file download page payload
An admin save of the File download page design sent a body the API rejects — not a visitor download error.
Why this happens
PATCH to the file-download design route expects JSON like { "config": { … } }. Missing config, non-JSON, or wrong shape triggers this. Deeper config parse failures may return different 500 messages.
Diagnose and fix
1. Open Admin → Design → Downloads → File download, refresh, and save again via the UI.
2. Scripts must send Content-Type: application/json with a config object.
3. Check Network for the PATCH response details.
4. Same family as Invalid upload page payload / pre-download payload.