cannot create forum topic.

moreover, I get:
The vocabulary can not be modified in this way.
which comes from:

/**
 * Make sure incoming vids are free tagging enabled.
 */
function taxonomy_node_validate(&$node) {
  if ($node->taxonomy) {
    $terms = $node->taxonomy;
    if ($terms['tags']) {
      foreach ($terms['tags'] as $vid => $vid_value) {
        $vocabulary = taxonomy_get_vocabulary($vid);
        if (!$vocabulary->tags) {
          // see form_get_error $key = implode('][', $element['#parents']);
          // on why this is the key
          form_set_error("taxonomy][tags][$vid", t('The %name vocabulary can not be modified in this way.', array('%name' => $vocabulary->name)));
        }
      }
    }
  }
}

which is strange because $vocabulary->name is not set, and there is no free-tagging associated to the forum vocabulary anyway...

duplicate of: http://drupal.org/node/41484#comment-136109