Hello,
If it's working using PHP CLI mode, the most common issue is related to .config folder. We recommend you to check that this folder exists (create it if doesn't exist in the home folder of the web server user) and set 777 for the whole directory. And also check your web server logs and the 4th item on http://www.phpdocx.com/documentation/conversion-plugin/debugging-libreoffice
Regards.