The original login cookie was set for "www.intfiction.org" which would cause problems if you came from in on other URL (or a variation of that one, even). This was changed yesterday to blank, so it'll use whatever domain you actually come in on. However, because the old cookie is still there, I think that may be causing some people problems staying logged in now.
To fix, you should be able to open your cookies and delete any that are for www.intfiction.org.
In FireFox and MSIE you can view individual cookies, and delete just those without destroying your saved info from other sites. I'm not sure if this is possible in other browsers, but probably. Once you do this and then log in, I think you'll be fine.
Sorry for the inconvenience.