Strange problem with AJAX and mod_rewrite

Strange problem with AJAX and mod_rewrite

I'm really don't know what to do. I have similar forms on different pages. The first one is situated on http://www.site.com/new/register.php .
The second one is located at http://www.site.com/blogs/someones-blog/account . The second is modified by mod_rewrite, and physically that form has address http://www.site.com/new/blogs.php?user= ... on=account .
File ajaxengine.php which process data sent by that form throught AJAX is located at http://www.site.com/new/ajaxengine.php .
So, I have script on JS:
/////////////////
function validateBlogForm() {
        error=0;
        $.ajaxSetup( {
          url: "ajaxengine.php",
          global: false,
          type: "POST"
        } );
        if ($("#login").val()=="") {
                $("#login").css("border","1px #FF0000 dotted");
                $("#wlogin").empty();
                $("Write your login here.").appendTo("#wlogin");
                $("#wlogin").show("slow");
                error=1;
        }
        else {
                    response = $.ajax({
                        data: "mode=checklogin&login="+$("#login").val(),
                        error: function(xml,status,error){
                        alert('Error loading XML document(login)');
                    },
                    success: function(xml){
                        $(xml).find('login').each(function(){
                                var login_text = $(this).text();
                                        if (login_text=="busy") {
                                                $("#login").css("border","1px #FF0000 dotted");
                                                $("#wlogin").empty();
                                                $("This login is alredy in use. Try another.").appendTo("#wlogin");
                                                $("#wlogin").show("slow");
                                                error=1;
                                        }
                                        else {
                                                $("#login").css("border","1px #000000 solid");
                                                $("#wlogin").hide();
                                        }
                            });
                    }
                });
        }
        if (error!=0)
                return false;
        return true;
}[/i]

In ajaxengine.php I have this code:
[i]header('Content-Type: text/xml; charset=windows-1251');
switch ($_REQUEST["mode"]) {
  case "checklogin":
  $login = addslashes($_REQUEST["login"]);
  $res = mysql_query("SELECT * FROM blog_users WHERE login='{$login}'");
  if (mysql_num_rows($res)>0)
  echo '<login empty="no">busy</login>';
  else
  echo '<login empty="yes">empty</login>';
  break;
        ...
}


So, if I submit my form from www.site.com/new/register.php all scripts works completelly right. With no errors. But if I submit it from www.site.com/blogs/someones-blog/account the script show me error "Error loading XML document(login)".
First my thought was that script can't find ajaxengine.php at www.site.com/blogs/someones-blog/ , so I modified mod_rewrite, and I got access to ajaxengine.php from any place on my site.
Can someone help me to solve this problem?