Authentication or 'auth' failures on Windows happen for a number of reasons, either relating to the password itself or the device count you have been connected to.
- Try to change your password, log out from the NordVPN app, and then log back in. Change your password here: https://my.nordaccount.com/reset-password/.
- If the issue persists, make sure you have no more than 10 devices connected simultaneously with one account. If yes, disconnect one of the devices.
If that does not help, reinstalling the application should:
- Go to Control Panel > Uninstall a Program / Programs and Features.
- Locate "NordVPN", "NordVPN network TAP" and "NordVPN network TUN" instances and remove them if they appear on the list.
- Restart the computer.
- Go to https://nordvpn.com/download/, download the NordVPN app, and go through the setup steps.