In the Admin panel, edit your config settings. In the Path & URL Settings section, you'll see an entry that start with "If you are using cookies you can specify a path as well...." That's where you would specify a path for the cookies. [] The path you specify is actually a URL path, from the domain root. So, if threads is in
http://www.domain.com/forum/threads. Then you would set the cookiepath to /forum/threads. - Rick[/]
Note that by changing this it will log out everyone, since their cookies will no longer be valid. Other cookie-based settings will get reset as well.
I can't guarantee this will solve the problem, but it may help.