To fix this, you need to add EnableEventValidation="false" to your Page directive. This should be a FAQ for CascadingDropDown, but basically ASP.NET tries to make sure that postbacks don't contain stuff they shouldn't based on what it knows about the page it sent down. Well, CDD is all about sending back data that wasn't in the page to begin with, so this feature needs to be disabled for pages that use it.

That's the good news. :) The bad news (or better news depending on how you see it) is that this should have been easier to debug. CDD tries to populate the list with error details when an error occurs (see _onMethodError), but it doesn't because a change some time ago to use name/value pairs for the data type wasn't applied to the error methods. I've logged this for fixing in the next release.

But for now, your issue should be trivially solvable by flipping EnableEventValidation as I note above.

Thanks!