libreoffice headless listening mode - not working if not root

I’m trying to convert a docx file to pdf using LibreOffice headless.
I started the server in listening mode, cause it seems the way to go to use it concurrently and for performance benefits (I’m planning to use it in a web application) as:

libreoffice --headless --accept="socket,host=localhost,port=9997,tcpNoDelay=1;urp" --nodefault --nofirststartwizard --nolockcheck --nologo --norestore --invisible

however, as normal user, the following command does not work, without any output (errors or messages):

libreoffice --headless --convert-to pdf --infilter="Microsoft Word 2007/2010/2013 XML" LaP1.docx

running it as root, instead, it works.

Anybody experienced that and can provide any help?

I’m running Ubuntu 16.04.2 LTS Ubuntu 16.04.2 LTS.