A field technician applied a Group Policy setting to all the workstations in the network.

The workstations and the authentication server have a system clock difference. If a Group Policy setting is applied that forces the workstations to use a specific SNTP server, but the system clock on the workstations and the authentication server are out of sync, then this can cause authentication issues and users will be unable to log in. In this case, the most likely cause of the issue is a difference in system clocks and the technician should ensure that the clocks on the workstations and the authentication server are in sync.