ArgumentNullException in Send pipeline (Biztalk Server 2006) - aaron.ko

Hello,
I have a web port (send - receive) with passthru pipeline attached to
it. The sent message is very simple and consists of two string fields.
In my own PC (win xp)  there is no problem consuming that web service,
but while I deploy it to another server (win 2003) i got the following
exception while processing the send pipeline:

ArgumentNullException: String reference not set to an instance of a
String.
Parameter name: s

The funny thing is that I don't have any parameter or field called

I have compared two sent messages, from my PC and from the server, and
they are perfectly identical.
I have compared ports on both servers and they are identical too
(address, credentials, pipelines etc.).

So, any one has a clue what could go wrong?

Thanks.