administrator - Windows Defender turned off by Group Policy

My PC with Windows 10 Enterprise 2016 LTSB v.1607 installed, automatically restarted for an error (I don't remember the error, I just remember a blue page in Windows style with a sad emoticon face).
When the PC restarted, Windows defender was disabled and I'm not able to turn it on.
The error is:

Windows Defender is Turned off by Group Policy. To protect your computer
against harmful or potentially unwanted software, contact your system
administrator to enable Windows Defender via group policy.

I've tried all the following things:

1. Delete the value in the DisableAntiSpyware key in HKey_Local_Machine\Software\Policies\Microsoft\Windows Defender (I've not this key because my version is older than v.1703).


2. In Local Computer Policy > Administrative Templates > Windows Components > Windows Defender the option Turn off Windows Defender is already set to not configured.


3. In Services the service Windows Defender is not present.


4. In Control Panel > Action Center under the voice Windows Firewall I can read: For your security, some settings are managed by your system administrator, but I'm the administrator.

How can I turn on Windows Defender?

Answer

This sounds more like malware than an accidental change to Group Policy, since that switch is normally not used by AV suites to disable Defender. If you cannot reset that in GPE, then make bootable media to scan for malware, e.g. Avira Rescue System, Kaspersky Virus Removal Tool or Rescue Disk.

You might also try installing a third party AV suite, such as the free versions of Avast!, Avira or alternatives to see if that corrects the issue. You would then have the choice of staying with that AV suite or uninstalling it to see if Defender works again.