It's a bit weird, but every time I click on the "Forum" button at the top of the screen, I log out. I don't know if it's an error in my browser (I use Opera) or it's a bug of the website (don't worry, your site is still very good ), but this is a bit annoying. Obviously it happens not only if I click "Forum", but with every other buttons also.
Yeah, that sounds like a problem your browser is having with the forum software (PHPBB3) or the cookies it set.
Try clearing all of your cache and cookies from this website.