C# - JQuery AJAX - Error Status Code - 200 - Text "parserorro | OK"

I'm developing an ASP.Net web site using VS 2008 and .Net Framework 3.5, and I want to use jquery ajax in a test page, the code looks like this:

C# Method
[WebMethod]
public static string test()
{
return "Server Response" ;
}
$(document).ready(function() {
$("#myDiv").click(function() {
$.ajax({
type: "POST",
url: "AjaxTest.aspx/test",
data: "",
contentType: "application/json;charset=utf-8",
dataType: "json",
success: function(msg) {
// Replace the div's content with the page
// method's return.
alert(msg.d);
},
error: function(result){
alert("error occured. Status:" + result.status
+ ' --Status Text:' + result.statusText
+ " --Error Result:" + result);
}
});
});
});

So When I use Jquery 1.4.4 like this : I get : Status 200; Status Text: OK When I use Jquery 1.5 I get: Status 200; Status Text: Parsererror So I created a new WebSite in Visual Studio, copy and pased the code there, and it works fine !!!! I can't figure out what causes the problem. Also I have used methods with parameter, and setting data:"{}", and removing data completely, but nothing seems to work. I don't know if has to do anything with the DevExpress components that I'm using or not. I also found a good answer which was working with complete method like this :

complete: function(xhr, status) {
if (status === 'error' || !xhr.responseText) {
alert("Error");
}
else {
var data = xhr.responseText;
alert(data);
//...
}
}

But I don't know if it will work fine or there might be some other problem with this method too. I also don't know how to access response data from here.