Description: When you debug  an ASP.NET application in Visual Studio .NET, you may receive the following error  message:
"Error while trying to run  project: Unable to start  debugging on the web  server. server  side-error occurred  on sending  debug HTTP request."

Cause:
This problem can occur if there are syntax errors in the ASP.NET application's Web.config file.

Solution:
Check Extensible Markup Language (XML) in the Web.config file is valid and well formed.