JabRef version JabRef-4.0.0-dev--snapshot--2017-06-22--master--bee062c6a

I used the line from http://help.jabref.org/en/CommandLine
java -jar JabRef-4.0.0-dev--snapshot--2017-06-22--master--bee062c6a.jar -o EndokrinologieKunde.xml.docbook -n true EndokrinologieKunde.bib

Edit: There was a typo should read file.xml,docbook therefor I delete the noise