Removing Site Map Module
It sounds like you might have another module also implementing that path.

To check, install the Devel module (if you haven't already), and go to /devel/php. Put in this code:

foreach (module_implements('menu') as $module) {
  $items = module_invoke($module, 'menu');
  if (isset($items['sitemap'])) {
    print $module;
  }
}


And submit the form. If another module is directly responsible for the /sitemap path (i.e. it's not a redirect or something else), its name will be printed to the screen.