There is an error in XML document (1,2) , System.InvalidOperationException: <AuthorizationResult xlms:""> was not expected
It appears to be complaining about an unexpected root element and there is some confusion surrounding that elsewhere in your question.
In the question title you have <AuthorizationResult>
In the example XML response you have <AuthenticationResult>
In the C# you have [XmlRoot(ElementName="AuthorizationResult")]
At the end of your question you mention <AuthorizationData>
That's quite a few subtle variations on a theme.
Assuming the XML sample response is correct, you should change the C# to expect the root element to be <AuthenticationResult>...
[XmlRoot(ElementName="AuthenticationResult")]public class AuthenticationResult{ ...