While working among half a dozen SharePoint 2010 sites one day, spread through Internet Explorer 9 tabs, I was testing some security trimming on some new groups for a particular site. When I tried to sign in as another user, I was immediately given a blank white page that simply said 401 UNAUTHORIZED.
The only related information I could find on the web was a post referring to the error but without a suitable solution: http://www.3guysonsharepoint.com/?p=979
I fired up a session of Chrome and tried the same thing. The Login prompt was displayed and everything worked just fine. I returned to IE9 and tried to sign in as a different user on the other tabs I had open. All returned the 401 UNAUTHORIZED error.
I closed down all instances of IE9, cleared my cache, and fired it back up. This time all the sign in operations worked as expected. I'm not exactly sure which part of the IE session got out of whack, but starting fresh seemed to clear things up. Hopefully it's not a problem that will resurface