Use user session for the drupal_http_request requesting the library. By default, drupal_http_request runs in a sandbox environment, thus the request doesn't have any idea about the current user. This in turn means that the request on the library is performed as an anonymous user, who may not have appropriate credentials to access the library.

<div id="<?php print $library_id; ?>" class="dnd-library-wrapper">
  <div class="header">
    <?php print $header; ?>
  </div>
  <div class="library">
    <?php print $library; ?>
  </div>
  <div class="footer">
    <?php print $footer; ?>
  </div>
</div>