Java APIs

I hope anyone can help me with this:
I am trying to call a RMI server from a Client passing to natting firewalls, meaning my client has a private IP address and the server is installed on a node with a private ip address.
On the server side the firewall is set up to forward all calls on port 1099 to the private ip address of the server node.
So when I call the server from the client I use the public ip address of the firewall
I have done some network tracing and I can see first this stream:
TCP local ip:2002->public ip:1099 [SYN]
TCP public ip:1099->local ip:2002 [SYN, ACK]
TCP local ip:2002->public ip:1099 [ACK]
RMI local ip:2002->public ip:1099 JRMI, Version: 2, StreamProtocal
RMI public ip:1099->local ip:2002 JRMI, ProtocolAck
RMI local ip:2002->public ip:1099 Continuation
RMI local ip:2002->public ip:1099 JRMI, Call
RMI public ip:1099->local ip:2002 JRMI, ReturnData
TCP local ip:2002->public ip:1099 [ACK]
TCP local ip:2002->public ip:1099 [FIN, ACK]
TCP public ip:1099->local ip:2002 [ACK]
TCP public ip:1099->local ip:2002 [FIN, ACK]
TCP local ip:2002->public ip:1099 [ACK]

The connection is ended and the RMI client tries again on another port (this is build into the program).
No data is ever returned.

In the Protocol Ack I can see that the EndPointIdentifier is pointing to the public ip address of the client side firewall. In the continuation I see the client nodes private ip address.

Now, if I open a vpn connection to the remote network (the vpn ip address of my host is listed in the hosts file of the server node) then data is returned. Obviously they are returned on the vpn connection, but why?
The client vpn ip address is not on the same network as the client private ip address.

A network trace produces the same stream as above except for the connection being closed.

Can anyone explain the behaviour and - hopefully - point me in a direction how to solve this.

All help is most appreciated.

cwolfhagen