Are you using a bookmark, and if instead of logging in you just go into a thread, does it then show you as logged in? That happened to me because my bookmark was saved as http:// and but it needs to be https:// - if so you can edit the bookmark, or create a new one from a logged in page.
I'm having an odd issue. When I'm away for any length of time and come back (to the big board as in my bookmarks) it is showing me as logged out (and a note in the login box warns about the site not being secure). If I click on a thread, it takes me to an https secure page on which I'm already...