Pre-register the inner class as well. A cut paste

error from my originals.

jme3-networking/src/main/java/com/jme3/network/service/serializer/ClientSerializerRegistrationsService.java

@@ -55,6 +55,7 @@ public class ClientSerializerRegistrationsService extends AbstractClientService
         // This is the minimum we'd need just to be able to register
         // the rest... otherwise we can't even receive this message.
         Serializer.registerClass(SerializerRegistrationsMessage.class);
+        Serializer.registerClass(SerializerRegistrationsMessage.Registration.class);
         
         // Add our listener for that message type
         serviceManager.getClient().addMessageListener(this, SerializerRegistrationsMessage.class); 

jme3-networking/src/main/java/com/jme3/network/service/serializer/ServerSerializerRegistrationsService.java

@@ -51,6 +51,7 @@ public class ServerSerializerRegistrationsService extends AbstractHostedService
     protected void onInitialize( HostedServiceManager serviceManager ) {
         // Make sure our message type is registered
         Serializer.registerClass(SerializerRegistrationsMessage.class);
+        Serializer.registerClass(SerializerRegistrationsMessage.Registration.class);
     }
     
     @Override