I was working on development of a Silverlight control. The development was mostly converting an existing application to use new SL framework. When I tried to debug the application, I got hard exception thrown at me.
Invalid or malformed application: Check manifest
So I decided to look at AppManifest.xaml file that gets packed in XAP package. The way to do that is as follows:
I was not expecting anything wrong with this manifest file or the contents of the package itself. After some digging around I realized that I had changed default namespace of Silverlight application project and changed namespaces of all classes to match that as well. But I never changed the entry point of the application in project settings. So I brought up the properties dialog box for the project and fixed the Startup Object value and everything went to normal.