Spotted this issue via
MANTRUN-200. The reason is that in the DefaultClassRealmManager a new realm is created where the parent classLoader is null. This implies that the bootstrap classloader is used as parent.
With Java8 nashorn has become an extension and is not part of the bootstrap classloader anymore.
It is kind of strange that we want the bootstrap classloader here, it makes more sense if the system classloader is used (but with Java 7 and older versions of Java this was not an issue, both worked fine).