"Error occurred while trying to make a request to URI ... localhost:9009/EntityService.svc ..."

   Symptom:

Everything seems right. Maybe the application was working before and you don't remember making any changes.

You press F5, Cassini starts, the browser appears, the first application screen appears, it all looks good,  

� and you get the following error the moment the application tries to perform a service action (e.g., DevForce tries to log in):

An error occurred while trying to make a request to URI 'http://localhost:9009/EntityService.svc'. This could be due to attempting to access a service in a cross-domain way without a proper cross-domain policy in place, or a policy that is unsuitable for SOAP services. You may need to contact the owner of the service to publish a cross-domain policy file and to ensure it allows SOAP-related HTTP headers to be sent. Please see the inner exception for more details.

There is no inner exception.

The error message leads you on a wild goose chase.

Probable Cause:

The VS Start-up project has become the Silverlight project (e.g., ModelExplorer) instead of the Web project (e.g., ModelExplorerWeb). 

Who notices that Visual Studio has lost track of the start-up project? 

Who thinks of that when the app seems to launch perfectly?
Maddening.

Solution:

Make the Web Project the start-up project.