Hi Sami,
I want to make you couple recommendations:
1. I notice from the Web-Service response that you are using your EAI to make the request. I will recommend you to use SoapUI first.
2. Setup you End point and security in Ariba. Be sure the Web Service is enabled too.
3. Setup the same security in SopaUI.
Try and then post your results.
Regards,
Juan.