Error 401 is being triggered during successful logins
All user logins -- even successful logins -- involve at least one "401 Unauthorized" error code.
The first 401 error occurs when the user visits the password-protected site with his browser. The server responds with "401 Unauthorized", but the user's browser hides that first error message and instead brings up a Username/Password prompt. Then, each time the user submits the Username/Password prompt, the server will either return additional 401 Unauthorized errors, or it will allow the user access.
Guardian is stateless and so it is not possible for Guardian to defer notification of the 401 error until it sees whether the visitors logs in successfully. Many Guardian admins choose to disable all email notification of 401 errors, or they send their email notices to a special folder. In that way you can avoid being flooded with unimportant messages.
"Error 401 is being triggered during successful logins" https://www.xav.com/scripts/guardian/help/1019.html