An anonymous user id is what counts. Therefore, you must do it right. In this case, the application pool has an identifier that I want to use for the application, but this application did not actually use it.
In IIS Manager for IIS7, left-click on the application in the tree and click on the "Properties" tab at the bottom of the middle pane.
In the "IIS" section (I selected "Area" in the "Group By:" drop-down list in this area), find "Authentication" (or "Authentic ..." if the listview doesn't have focus).
Double click on it. "Anonymous authentication" must be enabled (it was).
Right-click "Anonymous Authentication" and select "Change ..." from the context menu.
Change "Anonymous User ID": from "Specific User:" ("IUSR") to "Application Pool Identifier" and click "OK."
Restart the web server.
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