Hello,
The conversion plugin based on LibreOffice needs read and write access to the .config directory on the PHP HOME.
If you don't know this HOME folder you can run
getenv("HOME");
to get it.
You can change this path in the PHP config file or dynamically running this PHP line:
putenv("HOME=/var/www");
before doing the conversion (we are setting /var/www but you can set any other folder).
Regards.