Can you show me the whole exception, e.g. if you catch it, then give me the output of Exception.ToString(). Does the stack trace really show it's within property KnownStacks?
It the exception really unhandled, or are you only seeing it because you're running in the debugger. As the device won't have the Microsoft stack, it will fail to load with an exception (though not that one) when the library tries to load it before going on to try loading the Widcomm stack.
It might help if you enable "reportAllErrors", see the "Stack configuration files" section in the Widcomm doc, i.e. copying the template to 32feet.config on your device and ensure it contains reportAllErrors="true".