Why would I get a 404 on "Authoring Information" autocomplete?
Most likely because your autocomplete isn't actually using the standard path for the user autocomplete.

From the image, your autocomplete is pointing to /login/autocomplete, a path that doesn't exist in a standard Drupal installation (or indeed one with about 200 contrib modules enabled, I just grepped a couple of large projects).

The correct path for the user autocomplete is /user/autocomplete.

Presumably there's some custom/contrib code changing the autocomplete to a different path, but failing to actually implement that path.

The best thing to do would be to search your codebase for the string login/autocomplete and go from there.

UPDATE

The change might be being made by the User Tweaks module:

function user_tweaks_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if (user_access('view login autocomplete')) {
    switch ($form_id) {
      case 'user_login_block':
      case 'user_login':
      case 'user_pass':
        $form['name']['#autocomplete_path'] = 'user/login/autocomplete';
        break;
    }
  }
}