tx:date not recognized as partial request

Using tx:date with JSF 2.x returns a wrong answer:
<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n{
"tobagoAjaxResponse": true,
"responseCode": 200,
"ajaxPart_0":

{ "ajaxId": "j_idt1:j_idt13:j_id42:j_id43popup", "html": "... snipped ...", "responseCode": 200 }

,
"jsfState": "<input type=\"hidden\" name=\"javax.faces.ViewState\" id=\"j_id1:javax.faces.ViewState:0\" value=\"-1186034638223701276:-8097948037232765006\" autocomplete=\"off\">"
}

As seen in example app it should return something like
{
"tobagoAjaxResponse": true,
"responseCode": 200,
"ajaxPart_0":

{ "ajaxId": "j_idt1:j_idt13:j_id42:j_id43popup", "html": "... snipped ...", "responseCode": 200 }

,
"jsfState": "<input type=\"hidden\" name=\"javax.faces.ViewState\" id=\"j_id1:javax.faces.ViewState:0\" value=\"-1186034638223701276:-8097948037232765006\" autocomplete=\"off\">"
}

The error seems to be
<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n
so the answer could not recognized as json answer