При выпадании следующей ошибки 

[InvalidOperationException: Operation is not valid due to the current state of the object.]

   System.Web.HttpValueCollection.ThrowIfMaxHttpCollectionKeysExceeded() ...

[HttpException (0x80004005): The URL-encoded form data is not valid.]

   System.Web.HttpRequest.FillInFormCollection() ...

В веб конфиге добавляем ключ <add key="aspnet:MaxHttpCollectionKeys" value="NUMBER" />

в секцию <appSettings>