View or Preview is blank page

This problem exists for a long time now, and I don’t know how to fix this.

View or Preview gives a blank page. The only source code in it is:

<!-- ngg_resource_manager_marker --><script type='text/javascript' src='https://www.xxxx.com/xxxx/wp-includes/js/jquery/jquery.js?ver=1.11.3'></script>
<script type='text/javascript' src='https://www.xxxx.com/xxxx/wp-includes/js/jquery/jquery-migrate.min.js?ver=1.2.1'></script>
<script type='text/javascript' src='https://platform.tumblr.com/v1/share.js?ver=4.4.1'></script>
<script type='text/javascript' src='https://www.xxxx.com/xxxx/wp-content/plugins/wpaudio-mp3-player/wpaudio.min.js?ver=3.1'></script>

  <div id="fb-root"></div>
  <script>(function(d, s, id) {
    var js, fjs = d.getElementsByTagName(s)[0];
    if (d.getElementById(id)) return;
    js = d.createElement(s); js.id = id;
    js.src = "//connect.facebook.net/en_US/sdk.js#xfbml=1&version=v2.0";
    fjs.parentNode.insertBefore(js, fjs);
  }(document, 'script', 'facebook-jssdk'));</script>

I use the free Sendit version 2.3.8.

https://www.ads-software.com/plugins/sendit/

[ No bumping please. ]