I use asp.net cookie in my application here is my web config :

<authentication mode="Forms">
  <forms path="/" 
         defaultUrl="Default.aspx" 
         loginUrl="Login.aspx" 
         name=".ASPXAUTH" 
         slidingExpiration="true" 
         timeout="3000"
         domain="www.mysite.com"
         cookieless="UseDeviceProfile"/>
</authentication>

it works fine but I have a problem, after some days when a user has been working with the site application, suddenly my login control didn't work. I found out it will work after deleting temporary files.

Edit : Please pay attention to domain when User request www.mysite.com every thing is okay but without "www" login doesn't work. in firefox they are working very good. this is IE problem.

How I can solve this ?