Understanding Jooma Session and the Chronoform Authenticator

Hi everybody.

I created a simple form with Chronoform 4.
--- On the Preview Tab i have only a TexBox and a Submit Button.
--- On the Events Tab i have:
[list]Authenticator ( Allowed groups:Registered | Allow guests: No | Inheritable Permissions
: No | Denied groups: None) with Show Html into Allowed and Redirect User (index.php) into Denied.[/list]

On Submit Event I open in the same tab an external php file.

//define constant
if(!defined('_JEXEC'))
    define( '_JEXEC', 1 );
//you need to adjust joomla path according to your joomla installation
if(!defined('JPATH_BASE'))
    define( 'JPATH_BASE', "..................");
//include joomla core files
require_once( JPATH_BASE . '/includes/defines.php' );
require_once( JPATH_BASE . '/includes/framework.php' );
require_once( JPATH_BASE . '/libraries/joomla/factory.php' );

$mainframe = JFactory::getApplication('site');

    //or import plugin helper
    JLoader::import('joomla.plugin.helper');

    //to test whether some plugins is enabled
    $pluginenabled = JPluginHelper::isEnabled('authentication', 'joomla');

    $user = JFactory::getUser();

    if ($user->guest == 1)
    {
          //Redirect
    }
    else
    {
         //Create dinamically a page with a button 
         (click handler pointing [ by window.location.href "url"]  to the joomla page containing the form above is added to the button)
     }

Every time i click on the button of the second page and back to joomla, the autenticator failed and a login window is shown.

If i change the handler function: using window.history.back() instead of window.location.href it works and the form is shown.

I'm sure i'm wrong with Jooma Session but I don't understand what.

Can you help me to understand?

Thank you