Users are unable to successfully authenticate at preboot.
You see the following error after providing your authentication details:
Unknown User
The problem is reported after assigning a group to an individual computer.
A subsequent check via the ePO console identifies that the system to which the group is assigned has no users assigned.
NOTE: To verify whether the user is assigned to the computer, look at the Save Machine info file on the client. To enable the ability to save the file, you must enable the following option in ePO under Product settings, Allow users to create endpoint info file.