Supplier Invoice Web Service Via ABSL --- Tax Node Action Code

I'm getting the same problem as in https://answers.sap.com/questions/12880279/supplier-invoice-creation-via-web-service-from-abs.html --- the exact same data works in SoapUI but not in ABSL. SAP say that it's the fault of the solution even though "01" is a valid action code and are refusing to help further, so I'm turning to the community. Did anyone manage to solve an issue like this?

Lewis