Use the same Hibernate version on both.
i am using netbeans 5.5 with hibernate3 and facing the same problem when trying to use the find method of EntityMananger.
what is the solution to this problem???
finally found the solution.
earlier i just added the hibernate-entitymanager.jar, javaassist.jar to the class path of the entire application not individually and i was also not adding the hibernate3.jar.
now i included hibernate-entitymanager.jar, javaassist.jar and hibernate3.jar to the class path of both the client and ejb module separately.
then deleted all the old jar files, emptied the tmp folder, restarted the server, rebuilt all the packages, redeployed the application and executed the client. it worked fine!