[@matthias70 Participant 11 years, 8 months ago]

Found the solution.
You have to add this code to themes function.php to set up lead_topic

function custom_bbp_show_lead_topic( $show_lead ) {
  $show_lead[] = 'true';
  return $show_lead;
}

add_filter('bbp_show_lead_topic', 'custom_bbp_show_lead_topic' );