I realise I'm a little bit late to the game, but just wanted to let you know about what I discovered with SoapUI NG 1.7 when I experienced the same symptoms of WSDL failing over HTTP but working from file on disk. It might give you some clues to look at in the Open Source version (unsure exactly what is shared across the two products).
I found that the add/update WSDL over HTTP functionality always assumes that the HTTP response is a 200 OK, and processes the response expecting it as a WSDL document regardless. Thus if for whatever reason the HTTP response is something else, e.g. "401 Unauthorized" it still tries to process the response as a WSDL doc, and thus raising unexpected errors (raised as SmartBear defect number SOAP-5946).
As a side note, in my case due to this bug in Java, SoapUI was assuming the wsdl in question was at an external address and tried to use our web proxy, resulting in "407 Proxy Authentication Required", I worked around this by turning the SoapUI proxy functionality off as all of our use is internal.
