I have a similar issue. I get the Invalid remote object when I have a stateful session bean holding an object that has a stateful session bean reference in it. What triggers this is the Passivation of the outer EJB. as the server passivates the bean it chokes on the reference to the other stateful session bean.
I know that perhaps it is a naming issue or RMI/IIOP issue. Invalid remote object implies that the mapping or communication mechanism between stub and proxy is not correct. But with the way that Jboss is designed it does not need any stubs that are normally created for the references.