Unexpected Windows Server 2016 Shutdown: winlogon, NT AUTHORITY\SYSTEM, 0x500ff

The Reason Code says that it's a BlueScreen (SHTDN_REASON_MAJOR_SYSTEM | SHTDN_REASON_MINOR_BLUESCREEN)

Reference: https://docs.microsoft.com/fr-fr/windows/desktop/Shutdown/system-shutdown-reason-codes

You should check that your drivers/softwares are up-to-date. Don't forget to check your antivirus too, because it's possible that an outdated third-party antivirus can lead to bluescreens.

You can use BlueScreenView to help you analyzing BSOD memory dumps (if any).