Why displaying different page instead of set as startup page in asp.net

I started to work in a new project developed in asp.net. In this application authencation used is forms authentication and session timeout is 10 min. Also IIS used is IIS expression for development purpose.

Issue is that even if I set login.aspx as the startup page for the first time it goes always first to pagenotfound.aspx page. But in 2nd attempte it display this page correctly. Also if I click on any other page even 2 minutes keeping system idle it again display pagenotfound.aspx page but in 2nd attempt display page correctly. pagenotfound is the page which is created to display if user do any invalid request but unable to find the reason while it always goes to that page instead of startup or requested page