My whole HTML/JSP is pretty standard with JSTL tags to help with databinding while I send off JSON via JS. Submit the form data as JSOn via Fetch and off it goes to the controller with @RequestBody annotations and a RedirectAttributes redirect. Every part of my layers within the application is working as it needs to. DTO, Service, Exceptions and so on. Once I actually try to addFlashAttributes to redirect to another URL, I get status code 400.