Only since recently this problem exists, i noticed it yesterday. When pressing the "add reply" button while being logged off, you have the option to log in and then it takes you to writing your reply. Only now, instead of reaching the reply page there's an error, something like "you don't have permission to view this topic". So i have to click the topic again, then add reply.
See you .

happens to me too. but only seems to when i start a reply by pressing 'quote'

Fixed.