1.2.6.201312211419, Yes, and Yes.
I did find that if I disabled the plugin in my UpdateSite, and instead pulled the jars into my database in WebContent/WEB-INF/lib, the same XAgent throws an Error 500. If I remove the log4j jar from that same folder, it then runs and gets past the 'org not found' error, but then throws a: java.lang.reflect.InvocationTargetException
Domino server is 9.0.1, if that makes a difference on the various versions.