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. See this KB for reference.
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, please refer to the KB above.
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