I got this error and searched this community after problems with mocking a SOAP server. Saw the question having been asked before, but no clear answer.
Maybe my research has given me an idea what is going on.
I had made the server call like this: http//:<server:port>\<path> and got the error. Note the "\"! After correcting to http://<server:port>/<path> it worked as it should.
My guess is that SoapUI tried my URL and basically told me "there is another mockservice running - not the one you are trying". Simply: wrong URL!