Do you have cookies blocked, or have any security / cookie related add-ons installed?
Did you try deleting any cookies you have for larian.com and see if that would let you stay logged in when they were recreated?
If you stay logged in long enough, clicking My Stuff and then Cookies should bring you to a page where you can delete the forum cookies.
If that doesn't help you may need to delete your browser cache (that fixed this issue on a different site for me awhile ago; I first deleted cookies for the problem site as well as anything I didn't recognize or knew I didn't need to keep settings for).
