You are using phpDocX libs and get an unexpected error :

PHP Fatal error:  Class 'tidy' not found in /var/www/epub_converter/library/phpDocx/classes/TransformDoc.inc.php on line 308

You are parsing all lib files but no trace of a tidy class ?

This is normal, tidy is a php5 module, to add it, run this command:

sudo apt-get install php5-tidy