It seems that this is a very generic error, but if you examine the SOAPUI logs you can usually find out what’s causing it. The SOAPUI error log (found in the “soapUI Logs” in the eclipse soapUI perspective tabs) shows the error details.
In my case, there was a SchemaException: Error loading schema types. My WSDL referenced numerous .xsd schemas, one of which was on a secured remote site. I changed the schema reference to a local address using the ‘schemaLocation=../localpath/some.xsd’ attribute of the import tag and all worked fine.