This is the response in SOAP:
So it looks like a normal response, but if you check the specifications, it is a text/xml and Mendix can not handle this when called in a microflow:
Unfortunately it is a general web service, called by multiple other companies as well. So we do not really have an influence on the web service specifications.
I would inspect the result you got in SOAPUI. Because what Mendix is telling you is that it expected an xml response but got something different instead.
Regards,
Ronald