Preview not working: shows download error code

The Preview is not working. It displays a long page of code that starts with:
<div class=”error” style=”clear:both;”>
<div id=”mdocs-error”>
<p><b>Memphis Error: </b>Something when wrong, and your download has failed.</p>
</div>…..

– I read the Notes and tried the suggestions. Disabled Bootstrap and changed to a theme that had worked before. No changes.

– http://toni-esl.com/docs/

Thank you

https://wordpress.org/plugins/memphis-documents-library/