Invalid JSON body
The server could not JSON.parse the request body (or equivalent). Returned before field validation — common on admin/settings, checkout, upload init, and many POST/PATCH routes.
Why this happens
Empty body, truncated upload, wrong Content-Type with non-JSON payload, or a client sending form-urlencoded where JSON is required. Distinct from Invalid input (JSON parsed, schema failed).
Diagnose and fix
1. Retry from the official UI after a hard refresh (usually a transient client glitch).
2. Custom integrations: send Content-Type: application/json with a valid JSON object.
3. Watch proxies that strip or alter bodies.
4. If CSRF also fails, fix token/cookies first — some clients abort mid-build.