If you set the trace option to true, you can grab the XML that the client is sending. But I think you'll probably need some kind of sniffer to grab whatever the server response with when the exception is thrown.
But examining those may give some insight.
I recently made a PayPal Pro API using PHP 5's SOAP extension, and for me, it took a little bit to figure out how to correctly structure the array passed into the SOAP call so that the XML would be built correctly. As you said earlier, documentation is pretty thin.