This is normally caused by conflicts with jars
in your classpath or jre/lib/ext
Thanks for the tip Adrian,
it wasnt my CLASSPATH but I forgot I put a custom jar in $JAVA_HOME/jre/lib/ext that I also put in $JBOSS_HOME/lib/ext. And apparently jboss doesnt like that.
(Difficult to track the error though since jboss says he cant find javax/ejb/EntityBean)