Keep getting 'not-xml' response

Hi Guys,

I hope you can help me, although I'm not sure this is a directly SoapUI related issue.
I'm running a specific xml request but although it is completed succ essfully, passing the assertions and actually performing the respected action, the returned response contains only: <not-xml/> (See attached image)

Note:

I'm working on SoapUI Pro, and using an Excel data source from which the xml request takes one of it's tags values.

Can you please tell me if I'm doing something wrong, or you have an idea why this is happening.

Thank you all in advance.