This is indeed reproduceable in 5.17.0.
I tried with online tools, they also give "Error cannot find parameter". So its not just Mendix. Can you verify with the publisher if this is indeed working?
http://wsdlbrowser.com/soapclient?wsdlurl=http%3A%2F%2Fwww.hypotheekbond.nl%2Fsoapservices%3Fwsdl&functionname=getSignals