Hello,
I think I have a problem with the BinaryImporter / BinaryExporter.
There is a class witch extends com.jme.scene.Node and implements com.jme.util.export.Savable.
package my_package;
public class MyClass extends Node implements Savable
{
It works.
There was a problem with my classpath.
com.jme.util.export.binary.BinaryImporter#load( InputStream, ReadListener ) catches the exception which was thrown by com.jme.util.export.binary.BinaryClassLoader#fromName( String, InputCapsule ).
Attention: The HashMap used in com.jme.util.export.binary.BinaryClassLoader needs access to to your classpath. Yeah, no surprise, I know. 