Thank you Alessio and Juergen for the suggestions, you put me on the right track! :) Yes, I was using the jdk6 distribution of the application server and the endorsed mechanism, but the client was using jars from the %JBOSS_HOME%/client folder. As soon as the client used the jars from the /lib/endorsed folder, the error disappeared. Just out of curiosity, what is the difference between the /client and the /lib/endorsed jars? The files seem to have exactly the same size, which is why I assumed they were identical. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4188996#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...