No problem for the logins. We try to make things easy around here, and understand that users can get 2 accounts depending on how they register. That Login With Facebook button is just so tempting to click
Anyways... I just got into your site. The problem seemed to be your System - Expiresheader plugin. That was doing basically what I mentioned above. It was adding a header to each page of your site telling the browser to cache the page for 2 hours. Then, when someone logged in, the browser would just pull the non-logged in page from cache and display it. If you do a "Refresh" of the page in the browser, that will fetch the 'correct' logged in view.
Of course, please test, and let us know how it goes. If you run into any other issues, just let us know. We'd recommend leaving that off. You can always try to re-enable anything else you may have disabled in the process, but go slow and test along the way!
Finally, for the Login With FB button.. is that where you want it? We can gladly help you fix it there, but I'm assuming that's not it's final destination.. so get it where you want it, and we can go from there!
Thanks,
Alex