An update for the McAfee Endpoint Security (ENS) security software was released today that caused major headaches for system administrators all over the world as it prevented users from being able to login to their computers. This was a bad McAfee ENS Exploit Prevention content update that was causing issues with older versions of ENS. To fix this issue, McAfee quickly released definition update 9419, which prevented new workdstations from experiencing this issue. This fix would have to be done via Safe Mode, which as you can imagine, would be a royal pain for an organization with thousands of workstations.
Source: https://www.bleepingcomputer.com/news/security/bad-mcafee-exploit-prevention-update-blocked-windows-logins/