[_ck_]

Your bbpress htaccess in /forum/ looks fine EXCEPT for what I hope is just a manual typo

<ifmodule mod_rewrite.c>

case matters in htaccess if I recall correctly

It should be

<IfModule mod_rewrite.c>

note the IfModule

(eta: the apache manual does say “Directives in the configuration files are case-insensitive” but give it a shot anyway – bbPress doesn’t generate it like that so I am not sure why it’s lower case in your setup?)

If changing that does nothing, the htaccess in the webroot that runs wordpress is also bit messy and my bet would be it’s got rules conflicting with bbpress’s

Is your site active or can you temporarily rename/delete the htaccess in the webroot and see if suddenly the bbpress slugs start working correctly? MAKE SURE YOU BACKUP THE FILE FIRST.

ps. next time use something like pastebin.com for that long of a copy/paste and just post the link