Hello,
It depends on your web server config.
As you can read on http://www.phpdocx.com/documentation/conversion-plugin/debugging-libreoffice (4th item), the directory .config must exists with the right permissions in the root folder of the user that is running the script.
For example, the default config Apache is /var/www in a Linux server, so the correct path is /var/www/.config ; but this may vary for other web servers.
We recommend you to check your web server config to know the root folder of the user's web server or print it using PHP; and check your server logs to check any writing error to any folder.
Regards.