LoopbackCheck caused 401 only happens when you are browsing a local Web site using the fully qualified domain name (FQDN) or a custom host header and the web site is using Integrated Authentication.
It is common to use System.Net.HttpWebRequest in a web application to connect to a web service and if the HttpWebRequest is hitting local web service and conditions mentioned above met, LoopbackCheck will also cause 401 for the System.Net.HttpWebRequest too.
To address LoopbackCheck caused 401 issue, use one of the two options below.
Re-enable the behavior that exists in Windows Server 2003 by setting the DisableLoopbackCheck registry entry in the
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa registry subkey to 1. To set the DisableLoopbackCheck registry entry to 1, follow these steps on the client computer:
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