Hard to say without seeing all of the rules you created to mirror the forum
with, but instead of the use of the single word 'logout' as the rule maybe try
a partial full link instead.

for example if the logout url were this:

<http://www.somewebsite.com/forum/login.php?logout=true&sid=a829d4cd332c2e44fb0c887c365eb273>

maybe try something like this rule:

-http://www.somewebsite.com/forum/login.php?logout*

or

-*www.somewebsite.com/forum/login.php?logout*