Login page loops and reloads indefinitely

The Problem

From time to time, various users have been unable to log in, reporting that the login page seems to get caught in some sort of redirect loop, redirecting from the login page to the site back to the login page. This pattern either continues endlessly or until an error message is displayed.

What's happening

If the app was accessed over http, the login page reloads under https to securely prompt for credentials.  When the user logs in, an authentication cookie is sent to the browser.  The login page now attempts to redirect to the app, switching back to http.  Here's the problem.  The app can't find the authentication cookie if the cookie was stored by the browser in a different "integrity store" (that's what Microsoft calls it).  Why would the cookie be stored in a different integrity store?  Because Internet Explorer keeps cookies for sites running in protected mode separate from sites that aren't running in protected mode.  By default, sites that fall under Internet Explorer's security zones for Intranet Sites and Trusted Sites run with protected mode off.  Correspondingly, sites that fall under Internet Explorer's security zones for Internet Sites and Restricted Sites run with protected mode on.  

To see what zone a site is running in and to see if the site is in protected mode, right-click on the web page and choose 'properties'.

What's Wrong

It's possible that UniversitySite was added to Trusted Sites or Intranet Sites for http but not for https or vice versa.

The Fix

Look through Internet Explorer's Trusted Sites and then look through Intranet Sites.  Remove any entries to UniversitySite.  Add an entry like this to Trusted Sites:

*.universitysite.com

More Reading

Read more about integrity stores and this kind of problem in this  article in Microsoft's IE Developer blog.