As you mentioned that : "Problem adding a external lib jar file to jboss6.4.0eap"
So can you please let us know more about the following things:
1). Where did you place the new JAR file?
2). Are you creating JBoss Custom modules for placing your JARs ?
3). Which "external lib jar file" you added and In which directory? Inside the application or inside JBoss "modules" directory ?
4). Can you please share the complete StackTrace from the server.log where we can see the "Caused By"
5). IN the newly added JAR do you have any class which is already present in any of your Application Lib (Like EAR/lib or WEB-INF/lib) or JBoss modules?