Basic CRM integration question - cannot "insertRecords" via the API - error 4600

Hi everyone

I'm trying to do implement a very basic version of a "Add lead" form and am stuck. I tried quite a lot a variants, tried making api calls with the Firefox extension "poster" (https://addons.mozilla.org/en-US/firefox/addon/poster/) and so on but always get the error message:

<response><error><code>4600</code><message>Unable to process your request. Please verify if the name and value is appropriate for the "xmlData" parameter.</message></error></response>

This is the code I'm using, you should be able to copy/paste it and try it yourself.
I even added a hard coded block of xml instead of using the form but I simply don't get it to work, any ideas?

<!DOCTYPE html>
<html>
<head>
    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
    <script src="https://s3.amazonaws.com/hayageek/libs/jquery/bootstrap.min.js"></script>
    <link href="https://s3.amazonaws.com/hayageek/libs/jquery/bootstrap.min.css" rel="stylesheet">
</head>
<body>
    <form name="ajaxform" id="ajaxform" action="" method="POST">
        First Name: <input type="text" name="fname" value ="FIRST"/> <br/>
        Last Name: <input type="text" name="lname" value ="LAST" /> <br/>
        Email : <input type="text" name="email" value="EMAIL"/> <br/>
    </form>
    Output:<br/>
    <button class="btn btn-info" id="simple-post">Run Code</button>
    <div id="simple-msg"></div>
    <script>
     $("#simple-post").click(function()
    {
        $("#ajaxform").submit(function(e)
        {
            // NOT using the form at the moment
            // var postData = $(this).serializeArray();
            // var xmlObject = $('<Leads><row no="1"></row></Leads>');
            // $.each( postData, function( i, field ) {
            //    xmlObject.find('row').first().append('<FL val="'+field.name+'">'+field.value+'</FL>');
            // });
            var data = {
                xmlData:
                    '<?xml version="1.0" encoding="UTF-8" ?>' +
                    '<Leads>' +
                    '<row no="1">' +
                    '<FL val="Last Name"><![CDATA[TEST]]></FL>' +
                    '<FL val="Email"><![CDATA[test@test.com]]></FL>' +
                    '</row>' +
                    '</Leads>' +
                    '</xml>'
            };
            $.ajax(
            {
                url : "https://crm.zoho.com/crm/private/xml/Leads/insertRecords?authtoken=MYAPIKEY&scope=crmapi",
                type: "POST",
                data: data,
                success:function(data, textStatus, jqXHR)
                {
                    $("#simple-msg").html('<pre><code class="prettyprint">'+data+'</code></pre>');
                },
                error: function(jqXHR, textStatus, errorThrown)
                {
                    $("#simple-msg").html('<pre><code class="prettyprint">AJAX Request Failed<br/> textStatus='+textStatus+', errorThrown='+errorThrown+'</code></pre>');
                }
            });
            e.preventDefault();
        });
        $("#ajaxform").submit();
    });
    </script>
</body>
</html>