Ok, can reproduce it consistently in the default theme (i.e. when not logged in).
Logging in from the board index (either at the top right or by clicking 'login' at the bottom left) works.
Clicking on the 'general' forum (taking you to viewforum.php?f=4
) and then logging in using the username/password form on the top right (with or without the keep-me-logged-in tickbox) doesn't work.