The server-side error is actually caught and handled correctly. As per the RESTEasy guide on exception handling, a HTTP 400 is returned. Initially a JSONParseException is thrown in the context of a MessageBodyReader and is thus wrapped inside a ReaderException instance; this ReaderException is translated into a HTTP 400 response, except that the body is HTML which cannot be processed correctly in the client-side.

The client therefore needs fixing to either send the correct Accept headers or to not make assumptions about the content type of the response.