Ypu can not return a generic list. this is .net specific. I have developed alot of Web services used by thrid parties and I always had to change a list to an array. Which is easy to do to be used by another language system.
myList.ToArray().
The XML deserializer code generated by the VBA Web Reference tool is very literal about the order and format of the XML it consumes (based on the WSDL it was generated from). When one of your lists in the result object is empty, the server isn't generating a container element for it, and the VBA deserializer isn't prepared to handle the missing element. Try marking both lists in the Result class (on the service side) with [XmlArray(IsNullable=true)] - this will cause an empty container element to be generated with an xsi:nil attribute when one (or both) is empty, keeping the VBA deserializer happy.