I believe this is the same problem we've seen before with inner classes getting initialized before their parent classes. We originally had this issue reported for the SocketLibrary class when it was an inner class of RubySocket, and then had reports of other similar cases. It seems that after Java 6u18 on some platforms, referencing an inner class before ever referencing its outer class causes this error.
It may have been fixed in later releases, which could indicate why it doesn't show up for you (Tom).
The only fix we came up with was to move the inner class out or always reference the outer class first. The former is the cleaner fix. It would be a simple fix to make to jaffl.