System Proxy log warning failed to acquire user principal name from directory services 

When a domain user is authenticated on the Qlik Sense server, the authentication is done via AD at the server level.  Qlik Sense only does authorization of the user.

Even though Qlik Sense does authorization,  it still does a quick check to validate the user in AD via the LDAP server.  However, this check only takes place if a user exists in the Active Directory and is a domain user (DOMAIN\qv_service ) which is valid in this case.

The main point here is that when the Proxy server is trying to communicate with the LDAP server to validate the user,  if the communication is slow and does not get an immediate response , it still authorizes the user to successfully login based on the user existence in Qlik Sense repository but writes a warning in the proxy logs such as the warning below.

Everything still works and the user is authorized as expected and logged in.

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20180801T053718.389+1200    WARN    QLIKSERVER1    System.Proxy.Proxy.DefaultModules.Authentication.WindowsAuthenticationRequestHandler    31    2d499d60-cc9d-481d-94dd-e03f9c7a8ed4    DOMAIN\qv_service    Failed to acquire user principal name from directory services    The server is not operational.↵↓    at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)