HTTP 500 response on SFDC WebService via SystemAccessor

Hello all.

I have a new SFDC project, where I must make the Webservice call via Java Class using SystemAccessor.

When trying to make the call via "postman" I get HTTP 500 result along with XML payload explaining the fault:

However when running the interface via the Java Class, the SystemAccessor throws an exception and I cannot seem to be able to access the payload.

This is the code that I use to make the Webservice call:

Channel channel = LookupService.getChannel("Service_Name","Channel_Name");
SystemAccessor accessor = null;
accessor = LookupService.getSystemAccessor(channel);
InputStream in = new ByteArrayInputStream(PayloadXml.getBytes("UTF-8"));
Payload payload = LookupService.getXmlPayload(in);
Payload SOAPOutPayload = null;
SOAPOutPayload = accessor.call(payload);

Is there any way to extract the actuall SFDC faultstring as I need to use it for logging purposes?

Regards,

Imanuel Rahamim.