If you get a "HTTP Error 401.3 - Unauthorized" error when running a website using IIS one possible reason for that is that you have enabled "Anonymous Authentication" but the user running it is different from the one being set for the application pool.
To fix this, click on the website in question in IIS, double-click "Authentication", select "Anonymous Authentication" and click on "Edit" in the right pane. From here you can select "Application pool identity" (instead of the default "IUSR" user) if that option is available (which it should be in IIS 10 at least) or you can manually select the same user that is used for the application pool.
Now you should be good to go.