[@netweb Keymaster 12 years, 3 months ago]

bbPress will use that forum.php file as a wrapper template for all of bbPress pages and that is most likely what the issue is.

I’d suggest if you need a wrapper template to work with your theme you copy the contents of your themes page.php to a file named bbpress.php.

Then copy any bbPress templates you want to customize to your themes folder.

See these for more info on copying bbPress templates and child themes:

Functions files and child themes – explained !

https://codex.bbpress.org/theme-compatibility/