In an ArcGIS Enterprise environment configured with enterprise logins, when importing a database connection file to ArcGIS Server Manager to register the geodatabase, the user is prompted to log in. After entering valid credentials into Server Manager, the following error message is returned:
Error: Unable to Access the Authentication Service
This occurs when using a primary site administrator account or any other administrator level account to access ArcGIS Server Manager.
This error occurs due to the authentication settings on a federated ArcGIS Server where Windows authentication is enabled and Anonymous authentication is disabled on the server's Web Adaptor. For more information, refer to ArcGIS Help: Federate an ArcGIS Server site with your portal.
To circumvent this issue, disable Windows authentication and enable Anonymous authentication on the Web Adaptor of the federated ArcGIS Server.