org.omg.CORBA.MARSHAL when using callbacks with two chained gatekeepers

[Stefan Kochta]

I am trying to set up a chain of two gatekeepers (to overcome two
firewalls with CORBA) using the VisiBroker supplied with Borland
Enterprise Server 5.2: client -- gk1 -- gk2 -- server. Everythings
works fine until I try to enable bidirectional IIOP for callbacks. I
then get the following exception in the client:

org.omg.CORBA.MARSHAL: Cannot marshl non-local object for the
firewall. minor code: 0 completed: no

The client code is (approx.) as follows:

<snip>
// get server interface
AInterface aInterface = AInterfaceHelper.narrow( root.resolve( name )
);

// create client callback object (implizit activation)
AClientImpl aClientImpl = new AClientImpl();
AClient aClient = AClientHelper.narrow( poa.servant_to_reference(
aClientImpl ) );

// make call
aInterface.doCallback( aClient ); // <-- exception is thrown here
<snip>

Taking into consideration the fact that everything works fine until I
enable callbacks therefore I think that I may have misconfigured the
bidirectional IIOP ability. Right now I use the following settings:
server: vbroker.orb.enableBiDir=server
client: vbroker.orb.enableBiDir=client
gatekeeper1: vbroker.orb.enableBiDir=both
gatekeeper2: vbroker.orb.enableBiDir=both

Is there anything else I can configure?
Does anybody have a clue why I get the MARSHAL-Exception?

Thank you very much in advance,
Stefan.