Invalid upload page payload
This appears when an admin save of the Upload page design sends a body the API rejects — not when a member uploads a file.
Why this happens
PATCH /api/admin/design/download/upload expects JSON shaped like { "config": { … } }. If config is missing, the body is not JSON, or the shape fails validation, you get “Invalid upload page payload.”
Member chunked upload and Remote URL tabs do not use this message.
Diagnose and fix
1. Open Admin → Design → Download → Upload page (/admin/design/download/upload), refresh the page, and save again.
2. If you edited via API/script, send Content-Type: application/json with a config object.
3. Check the Network tab for the PATCH response details; fix the payload and retry.
4. If saves keep failing after a refresh, check app logs around the design upload route for Zod/parse errors.