Invalid pre-download page payload
An admin save of the Pre-download page design sent a body the API rejects.
Why this happens
Same shape rule as the file-download design route: PATCH must include { "config": { … } }. Wrong Content-Type or missing config triggers this before deeper parse.
Diagnose and fix
1. Admin → Design → Downloads → Pre-download — refresh and save via the UI.
2. Scripts: send a proper config object (HTML, continue button, and so on).
3. Check the PATCH response in Network.
4. Visitors never see this; they see the rendered pre-download wait page.