I just encountered your error the other day. This was on a call from Mx8 to KvK endpoint . It was working fine in Mx7.
What I did is find the certificate which is used by the endpoint, usually if you use your browser to go the wsdl en click on the lock in the address bar. There you can save the certificate in DER format. Save the certificate and upload locally in your Project > Settings > Certificates.
I think it has something to do with the Mendix (java) keystore which doesn't have this root certificate (yet).
An example from my solution:
I assume you do not need any certification, since you got it working in SOAPUI already. If you want to let the modeler know that it should not look for a client certification you can change your custom settings in the projectsettings.
To do that go to project module > settings > configurations > create a new config > Custom
Here you can set custom runtime settings. For this problem you will need to set the “NoClientCertificateUsages” (see also this link for more information and other options). As a value you will set the exact path to you webservice like this: Myfirstmodule.MyWebservice
Hope this helps!