java.rmi.UnmarshalException: error unmarshalling return; nested exception is:
Well - thats pretty clear. It looks like you do not have CalculatorImpl_Stub in a package. Therefore the JVM looks only in the "current directory" for the class file. You need to have all classes involved in this experiment in a package.
Probably it only works on a single machine because of where you start it.
subject: RMI between 2 JVMs in different physical machines