I noticed you linked to board.straightdope.com here. Are you sure you’re always going to the same domain? If you’re logged in at boards.straightdope.com, the cookie won’t be recognized at board.straightdope.com or ards.straightdope.com or any of the other URLs that come up in a Google search.
I have this problem with great frequency (although not every time). I often have to enter my name/password twice, or three times, or five or six times, before I actually get logged in. I’ve reported this in ATMB threads several times already.
Here’s one thread (01/14/2013) in which I reported this, and TubaDiva just thought my browser is corrupted:
But here’s another thread (02/17/2013) where a whole bunch of people report the same problem, but no mod or admin ever responded:
ETA: I use an old version of Firefox with an old version of Linux.
ETA-2: Given the overloading problem that we aren’t supposed to complain about anymore, one wonders: Is this authentication trouble just a symptom of the authentication server being overloaded?