I've figured out what the problem was for me: If I go to www . jacurutu . com, it will keep my login. But if I just go to plain jacurutu . com (which otherwise displays the same page), it will show me as not logged in (though if I then go on to www . jacurutu . com, I'll see that I still am). Presumably this has to do with the scope of the login cookie. Since I usually go to the page by typing into the address bar and choosing one of the auto-complete options, which has jacurutu . com as the first and www . jacurutu . com as the second, I was getting inconsistent behavior depending on which one I happened to choose.
Edit: The forum converts every URL to links, in the process wiping out the distinction I was trying to demonstrate. I've added spaces within the URLs to stop it.