Whenever I connect to a server I am booted within 1 minute with that error.
I have forwarded my ports and verified they are open. I have a stable connection and am able to play other games fine. A friend in one of the servers I tried to connect to reported seeing a “Could Not Authenticate User” error come up on his screen when I tried to join. I guess the two are linked.
This has been reported on the Steam and Bethesda forums, with no help what so ever.
http://forums.steampowered.com/forums/showthread.php?t=1886475