问什么因素会导致jquery.ajax每次都认为响应是错误的

EN

<script type="text/javascript">
jQuery("#FormButtonRun").click(function(event){
    submitEmailForm();
});

var submitEmailForm = function ()  {
    var submitdata = {'Email':jQuery('#FormEmail').val(), 'fName':jQuery('#FormName1').val(), 'lName':jQuery('#FormName2').val(), 'listnews':"Yes"};

    jQuery.ajax({
        type:'POST',
        url:'https://dev.formserver.com/ajax.php',
        data:submitdata,
        success: submitwrkd,
        error: submitfaild,
        dataType:'json'
    });
}

var submitwrkd = function(response, more, xhr){
    alert("It worked!);
    alert(response.msg);
}

var submitfaild = function(response, more, xhr){
    alert("allegedly that failed");
}

<?php
header('Content-type:application/json');
echo json_encode(array('success'=>true, 'msg'=>'It was a success'));
?>

POST /ajax.php HTTP/1.1
Host: dev.dataserver.org
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:51.0) Gecko/20100101 Firefox/51.0
Accept: application/json, text/javascript, */*; q=0.01
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer: https://dev.dataserver.org/popuptest
Content-Length: 127
Origin: https://dev.contentserver.org
Connection: keep-alive
Email={removed post data}

HTTP/1.1 200 OK
Server: nginx/1.8.1
Date: Tue, 16 May 2017 00:44:49 GMT
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST