Emilio,
I have met very similar problem, basically the same symptoms - did not have time to find the root cause because I was under pressure and once I have fixed it, I never came back to it - but maybe it can give you a clue:
The problem in my case was with the index of the SOAP variable. My webservice was based on ABAP function module. The web service external view, which I have seen in SE80 had a different sequence of output parameters than the actual WSDL, For Contact Center, the WSDL is important.
So this is what I did:
1) View the WSDL file in a browser (just copy&paste the address from your IVR SOAP element into a browser).
2) Find the output parameter name (EvPartnerGuidChar in your case) in the WSDL.
3) Check the position of the parameter in the parent <xsd:sequence>, then subtract 1 and use it as index in the IVR assign element.
Example:
My FM returns EvSegment, among other. This is on 2nd position in the WSDL:
And addressing it in IVR:
Have to do my homework and get deeper into Python sets and mapping to WSDL 
.
Hope this helps.
Regards,
Dawood.
Message was edited by: Dawood Hla Shwe One more hint - never put username and password to the SOAP element. A bit safer is to make the WSDL access anonymous in NetWeaver. Just run transaction SICF and create an external alias to your original WSD URL. There you can set your user and password used for anonymous access.