Hello,
If it’s working in Postman but not from Mendix, then the JSON structure is probably fine. The issue is more likely with the REST call configuration in Mendix.
The first thing I’d check is the headers, especially Content-Type: application/json and any authentication/authorization headers required by the backend.
Also, the error mentions the Anonymous user, so there’s a chance the request is being sent without proper authentication from Mendix, while Postman already has it configured.
Another thing to verify is whether the request body is actually being sent as raw JSON in the PUT call.
If possible, can you share:
That would help narrow it down pretty quickly.