I had a similar problem, and it turned out that the serializer could not distinguish between 2 classes I had with the same name (one was a subclass of the other). The inner exception looked like this:
'Types BaseNamespace.Class1' and 'BaseNamespace.SubNamespace.Class1' both use the XML type name, 'Class1', from namespace ''. Use XML attributes to specify a unique XML name and/or namespace for the type.
Where BaseNamespace.SubNamespace.Class1 is a subclass of BaseNamespace.Class1.
What I needed to do was add an attribute to one of the classes (I added to the base class):
Note: If you have more layers of classes you need to add an attribute to them as well.