• [ +] I have tested the latest master version from http://builds.jabref.org/master/ and the problem persists

JabRef 5.0-dev--snapshot--2018-08-24--master--f43b9c7e8
Windows 7 6.1 amd64
Java 1.8.0_181

There some tiny changes to make the exported xml compatible with docbook 5. I am most willing to provide these changes, but I do not know to whom. BTW I am impressed that the docbook export is now possible without stopping jabref and running the export from the command line.

Regards
Bernhard