Are you able to make the same Soap request successfully from outside of PowerCenter (e.g. cURL, SoapUI). This should help you start ruling things out.
You may follow two approaches:
- import the WSDL in SoapUI and create the request from scratch there
- enable "verbose data" mode for the session and use the "Soap Request" showing up in the session logs to carry out the same request from cURL
In addition to what Paolo suggested, try to change the WS security type to none and try once. Then configure application connection to pass user credentials. Refer to below article.