Ran into this error today while trying to build the docbook file for Refactoring Redmine. It looks like it’s a problem with Java IPv4 IPv6 networking on Debian. Simple solution is to edit /etc/sysctl.d/bindv6only.conf and change net.ipv6.bindv6only=1 to 0. Thanks to Roberto for the fix. Hope this helps someone. My error log: Validating your document... java.lang.NullPointerException …