If threads get too long, especially too many posts by 1 or people, then the forum software seems to "block" it for those users. I'm assuming some sort of flame-war prevention, but unfortunately I have no idea where it is set, or how to turn it off.
Try and keep threads short, and remove unnecessary posts if the thread gets long.
With regard to the root problem - I suspect you have
a) "Delete Session on Logout" set (Security Tab of Web Handler) and
b) Embed code in LoginForm that logs the user out.
Cheers
Bruce