Are you sure you are pointing to the same jbossall-client.jar that is available on the AS5 server?
Yes, the new jbossall-client.jar with the references only. I have a high inhibition threshold to post in this forum, so I check things like Class-Path several times ;-)
I found the problem: I've included my ejb-jar into my small test client to make available the EJB interfaces. Eclipse adds the ejb-jar dependencies to the Class-Path: the JBoss server libs. Seems to me, that server classes have overwritten the client classes.
Everything works fine, if I include the remote interfaces and client jars, only.
Stupid fault, sorry!