This may happen after you upload your project to the server. It may not load, and will show the following error:

This error occurs when you do not put your project into your web root (the /htdocs folder), but to some other folder within it. By default only the /htdocs folder is configured as an application in IIS, so to make your project run in a subfolder, you need to configure that folder as an application as well. The following article shows how to do it:

How can I create an application for my ASP.NET project?