How did you create the IS web service? Did you use SOAP-RPC or document/literal style? To what URL are you posting your soap request?
Assuming that you went with the path of least effort and used soap-rpc, you should check the IS service that you have exposed to ensure that it handling exceptions correctly. Try disabling all of the code that does real work and start by populating the output variables with hard-coded test values in a single map step. When that works you can switch back to the real code and see what happens.
Mark
#webMethods#API-Management#soa