I also had this same problem, the solution was to enable simple binding mode. Add the following into your external bindings file.
xmlns:xjc="http://java.sun.com/xml/ns/jaxb/xjc"
...
<jxb:globalBindings>
<xjc:simple />
</jxb:globalBindings>
Error is caused by the way JAXB is handling groups, as you've seen yourself.