I have just upgraded to IE11 and although it’s really awesome I had some weird issues with some MVC applications I am using. The problem occurred with the authentication cookie and the session id being stored in the URL. Although I explicitly enabled cookies for my custom applications’ URLs, the session id and the asp.net authentication token would still persist in the URL. This broke the POST requests as the server could not authenticate the request (the token was not posted in neither the URL nor a cookie) and would return a 401 response.
<authentication mode="Forms"><forms loginUrl="~/YourLoginUrl" timeout="2880" cookieless="UseCookies" />