Server Error in '/' Application

Symptom:
When trying to run a .NET web-application the following error is generated in the browser:

Server Error in '/' Application
Runtime Error
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Cause
This error has been observed when a new .NET web application has been copied to a server or renamed on the server.

Remedy:
Run "Internet Information Services (IIS) Manager" (this may be called simply "Internet Services Manager" on a workstation), this can be found under Control Panel > Administrative Tools.
Find the project in the list of projects. (Open up your computer in the list, and look under "Web Sites" - or "Default Web Site" if running on a workstation).
Bring up its properties (Right click > Properties).
If the 'application name' (under "Application Settings" on the "Directory" tab) is greyed out then click '[Create]' and then '[OK]'.