I have just encountered one issue while start the debugging one of my Silverlight 4 application. Silverlight 4.0 with VS 2010  was working fine few weeks back,  but today when I started debugging to test one of my application, I got the error message as below.

unableToDebug

I thought, It may be the problem with the Silverlight version which is selected for this project. I checked the Silvelight “Target Silverlight Version”  from the Project Properties. But the Silvelight version was correct.

Silverlight1

Then, I have also checked the associated hosting ASP.NET web application framework version. It was also targeting to .NET Framework 4.0 . So, there is no issue with my setup.

silverlight2

I started reinstalling the silverlight runtime and I received the below message :(

Silverlight3

I was bit confused and looking for some solution over internet, and finally got the resolution from the thread Unable to debug silverlight . ohh. It was due some wrong version installation. I installed the Silverlight 4 developer runtime from the mentioned link .

Silverlight4

After installation, the issue got resolved.  :)

But not sure, why there was the mismatch of silverlight runtime version as it was working fine few weeks back. :( . Sorry, I am not spending any time to investigate it right now! :)

Hope, This may help you and save your some amount of time.

Thanks !