PHP session is not configured

I am using session_start to create a session for the user when it is authenticated on the server, however the session is not being maintained in the page exchange, I am using AJAX requests to authenticate, then:

  

login.js + ajax - > login.php [create session if logged in] [message   error if not logged in] login.js - > [if logged in redirects   for panel.php]

The session is created in login.php (I tested with empty($_SESSION) ), but in page panel.php the session no longer exists, I used print_r($_SESSION) and the variable is not recognized.

Does anyone know the cause of the error?

login.js:

$(document).ready(function(){

$("#go").click(function(e){
    e.preventDefault();

    client = new clientSnotes("log");
    client.sendLogin($("#email").val(), $("#password").val(), "log");
});

});

functions used by login.js:

function clientSnotes(){

this.parseUser = function(data, log){
    var xml = $.parseXML(data);
    xml = $(xml).contents();

    var error = xml.attr('error');

    //No error
    if(error === '0'){
        var user = new userFields(xml.attr('id'), xml.attr('name'), xml.attr('email'), xml.attr('password'));
        window.location.href = "panel.php?" + user.getDataUrl();
    }
    else{
        $("#" + log).css("display", "inline");
        $("#" + log).html(xml.contents());
    }
};


this.assemblyRequisition = function(parser, url, data, error_log){

    $.ajax({
        type: 'post',
        url: url,
        data: data,
        success: function(data){
            parser(data, error_log);
        },
        error: function(){
            alert("Failed to send data to server.");
        }
    });
};

this.sendLogin = function(email, password, error_log){
    this.assemblyRequisition(this.parseUser, 'scripts_php/login.php', 'email=' + email + '&password=' + password, error_log);
};

login.php

        if(UserDataHandler::selectByEmail($_POST['email']) && UserDataHandler::selectByPassword($_POST['password'])){
        $user = UserDataHandler::selectByEmail($_POST['email'])[0];
        $assembler->addAttribute("error", "0");
        $assembler->addAttribute("id", $user->id);
        $assembler->addAttribute("name", $user->name);
        $assembler->addAttribute("email", $user->email);
        $assembler->addAttribute("password", $user->password);
        session_start();
        //testando
        if(empty($_SESSION))
            $assembler->addAttribute ("session", "no");
        else
            $assembler->addAttribute ("session", "ok");

        $_SESSION['con'] = 1;
    }
    ...
    echo $assembler->assembly("result");//escreve o resultado da operação na página

panel.php

        <div id="content">
        <?php
            print_r($_SESSION);
        ?>
    </div>