Online status doesn't appear

Seems like your $advforum_path isn't set in advanced_forum.module.

if (round((time()-$account->access)/60) < 15) {
       $vars['online_icon'] = theme_image($advforum_path . '/images/status_online.png', $alt = 'User is online', $title = 'User is online', $attributes = NULL, $getsize = TRUE);
    } else {
       $vars['online_icon'] = theme_image($advforum_path . '/images/status_offline.png', $alt = 'User is offline', $title = 'User is offline', $attributes = NULL, $getsize = TRUE);
    }

I think you assume $advforum_path is set after you call advanced_forum_add_css() but it isn't a global variable. All I did was added the line below before that code above:

$advforum_path = drupal_get_path('module', 'advanced_forum');