Error When Loading Xamarin Forms Control Library for Windows Phone

I've started getting this error for no apparent reason:

System.Runtime.Serialization.SerializationException occurred
Message: A first chance exception of type 'System.Runtime.Serialization.SerializationException' occurred in System.Runtime.Serialization.ni.dll
Additional information: Expecting element 'ArrayOfKeyValueOfstringanyType' from namespace 'http://schemas.microsoft.com/2003/10/Serialization/Arrays'.. Encountered 'None' with name '', namespace ''.

Before I crack open the serious debugging tools, has anyone seen this before

Thanks in anticipation]

Dean