Hello,
UPDATE: phpdocx 8 added DOCX to HTML native conversion:
https://www.phpdocx.com/documentation/introduction/word-to-html-PHP
Are you using the plugin based on OpenOffice or LibreOffice?
Please follow the instructions available on (http://www.phpdocx.com/documentation/conversion-plugin) to debug it.
The most common error is rw access to the temp folder access of any of both programs.
Regards.