Was it the java element in the RunServer target that threw the exception? The contents of the classpath element looks wrong in there - it should point at directories or JAR files, not at specific classes... besides - it looks like you are running from "myRMIServer.jar" directly - if you aren't using any classes outside of this JAR or the default Java classes that come with the JDK - you shouldn't need to set the classpath anyway.
Also, usually there is more information about why the build failed... that would help, too, if this doesn't fix your problem...
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