Weird, nice bug find.
Add this function to fix this until I can release an update with the fix in it:
add_filter('generate_blog_masonry','generate_disable_bbpress_masonry');
function generate_disable_bbpress_masonry()
{
if ( function_exists( 'is_bbpress' ) ) :
// If we're on bbPress, disable masonry
if ( is_bbpress() )
return 'false';
endif;
// Otherwise, enable it
return 'true';
}
How to add PHP: http://generatepress.com/knowledgebase/adding-php-functions/