WCF Error - An error occurred while receiving the HTTP response to http://localhost:50750/*******.svc - vb.net

Question

I am receiving the following error while debugging.
An error occurred while receiving the HTTP response to http://localhost:50750/FIGService.svc. This could be due to the service endpoint binding not using the HTTP protocol. This could also be due to an HTTP request context being aborted by the server (possibly due to the service shutting down). See server logs for more details.
Now I have seen many posts where people have had that error, I have cut my WCF down to absolute barebones and I am still getting the error, I am running the following:
Public Function TestXML(ByVal Username As String, ByVal Password As String, ByVal XML As String) As XmlDocument Implements FreshCloud.TestXML
Dim ReturnXMLDoc As New XmlDocument()
If ValidateLogin(Username, Password) <> False Then
ReturnString = "<FreshLead><Result><Message>Failed - XSD Validation</Message><DateTime>" & Date.Now.ToString & "</DateTime></Result></FreshLead>"
ReturnXMLDoc.LoadXml(ReturnString)
Return ReturnXMLDoc
End If
End Function
Don't worry about the uselessness of the code I literally just copy and pasted various bits to run a test, this is than ran with a simple test app with a button:
Dim client As FreshCloudClient = New FreshCloudClient()
Dim strXML = client.TestXML("ABC", "BS", "Doesn't Matter")
Dim strTest = strXML
client.Close()
I receive the error just returning the above, please any light that can be shed would be grateful this is a big project that I have had to sink my teeth into a Microsoft's vague error messages do not help a learner.
Cheers! :)
UPDATE
After digging through the logs for WCF I stumbled across this:
Type 'System.Xml.XmlDocument' is an invalid collection type since it does not have a valid Add method with parameter of type 'System.Object'.
Can anyone share with me why this exception would be thrown up by WCF?
Cheers.
UPDATE 2
OK so following Lerners advice I have now got the following:
Public Function TestXML(ByVal Username As String, ByVal Password As String, ByVal XML As String) As XElement Implements FreshCloud.TestXML
Dim ReturnXMLDoc
If ValidateLogin(Username, Password) <> False Then
ReturnString = "<FreshLead><Result><Message>Failed - XSD Validation</Message><DateTime>" & Date.Now.ToString & "</DateTime></Result></FreshLead>"
ReturnXMLDoc = XElement.Parse(ReturnString)
Return ReturnXMLDoc
End If
End Function
On my client side of have said to put the response to a variable and Console.WriteLine the variable and it just returned System.Object?
Surely this should return the XML as it shows in ReturnXMLDoc variable when I debug on the WCF side of things?
UPDATE 3
WORKING!
Lerner put me in the right ballpark and I just had to update the Service Definitions from within my 'Client' Application.
Cheers.

Answer 1

Return XElement instead of XDocument, XElement is IXmlSerializable.
var xml = XElement.Parse(ReturnString);
return xml;