IIS not recognizing .aspx extension (404 error)

While trying to install a .NET application on a Windows Server 2003 box, I ran into a problem where the .aspx extension was not being recognized. Browsing .NET pages resulted in 404 ("page not found") errors. Since the same set of application files had been installed on several other servers without incident, it seemed likely that there was something different...

.NET Error: httpRuntime requestValidationMode="2.0"

If you get .NET errors that reference It means that the application you are trying to run has set the property requestValidationMode to function the way it acted under .NET 2.0, but this directive only makes any sense under .NET 4.0 or greater, and your website is not set up to...

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IIS not recognizing .aspx extension (404 error)

.NET Error: httpRuntime requestValidationMode="2.0"