An error has occured on the server. For details, please check the event (application) log on the web server. Unable to retrieve ArcGIS install directory from the registry

In ArcGIS Server manager, you are unable to create a web app. You receive the error “An error has occured on the server. For details, please check the event (application) log on the web server. Unable to retrieve ArcGIS install directory from the registry.

The reason is because 32-bit IIS/ASP.NET is not enabled. We were running IIS7 on Windows Server 2008 64bit. Once you enabled 32-bit IIS/ASP.NET, everything works as expected.

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