onbeforeunload is blocking my page on IE 9 customized

I develop an application for a client who uses as browser 9 but customized, so the problem is that when I launch an action I gray screen to prevent the user from interacting until the action is finished . For that I use this script:

windows.onbeforeunload(function(){
    //my code to add div with style to block interactions  
});

on my browser it works perfectly and when the action ends the div is destroyed while at the client the div is never destroyed and thus the screen remains gray and creates a bug. thank you all