Workflow PDF 1.4 with NextCloud 19.0.3

Hi everyone,
I am new to Nextcloud. I need to covert docx files to pdf.
Therefore I installed the Workflow app Converter.

After struggling with Libreoffice preview path I dont know what the issue is.
Here what I have done.

  1. Installed Ubuntu 20.04 LTS
  2. Installed Nextcloud as a snap
  3. Setup https with custom certificate <- Works great
  4. Installed German Version of LibreOffice on ubuntu
  5. Changed config.php added preview_libreoffice_path

Now when I upload a docx file the logs show the following:
could not convert /admin/files/FileName.docx, reason: []

{“reqId”:“nHSZjgx9yn96GYFrf0eh”,“level”:4,“time”:“2020-09-29T10:37:42+00:00”,“remoteAddr”:"",“user”:"–",“app”:“workflow_pdf_converter”,“method”:"",“url”:"–",“message”:“could not convert /admin/files/fileName.docx, reason: []”,“userAgent”:"–",“version”:“”,“id”:“5f7311158c908”}

Is there anything I can get this thing up and running?