Error: Missing IDs

The APIs for creating new objects, documents or images are all throwing a “Missing IDs” error. This is with a 5.3.1 version hosted on a client environment that I am trying to access.

Even if I pass id:0 or no id (as below) the result is the same. At loss of ideas.

URL: /webservice/rest/object?apikey=
Request Method: POST
{
“id”:“1193”,
“className”: “TestClass”,
“parentId”: 1185,
“key”: “Page 1”,
“type”: “object”,
“elements”: [
{
“type”: “input”,
“value”: “Page 2”,
“name”: “title”,
“language”: null
}
]
}

The resolveId() seems to throw the error no matter what the input is.

protected function resolveId(Request $request, $id = null)
{
if (null !== $id) {
return $id;
}

    if ($id = $request->get('id')) {
        return $id;
    }

    throw new ResponseException($this->createErrorResponse('Missing ID'));
}

Any help or pointer is most appreciated. Must be somthing very silly but just not getting a handle on it.

Regards
K