unable to Second time login in my app

Hi Team, We developed application for iPhone, all pages / windows / screens working with navigation. Its working fine and we wrote code for Logout which is sussfully working good but we are unable to login second time after logout.

Here the code for logout button in help window.

btn_Logout.addEventListener('click',function(){ Approvals.cookie=null, navGroup.close(menuWin); // It is a menu window which will come next to login window navGroup.close(self); // It is a help window which will come next to menu screen });