it might be the case, that your "Stubs" and "Skeletons" do not fit together. The calling ejb must access a local interface implementation of the interface you are calling remotely. The "stub" has the same interface, or list of methods, as the remote object. The stub then forwards the request to the remote object via the remote "Skeleton" through RMI.
Check if your implementations of the interfaces are equal on both sides (local and remote). Practically this might imply that you have to deploy SBB if you made changes to SBA due to imcompatibilities after compilation.