Can't activate Virus & threat protection

May 26, 2018
Settings -> Update & Security -> Windows Security -> Virus & threat protection:

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When I click ok, everything is fine with a green mark, but "Virus & threat protection" is not fine.

When I click on "Virus & threat protection" I find there in red:

Your Virus & threat protection is managed by your organization.

So a couple of weeks ago, I tried disabling the antivirus and defender settings. But now I would like to enable this all, cause I've read about flaws in system protection leading to hacking & such. I am pretty sure that is because I have restricted either a service or change something in my regedit. The first thing that came to my mind was HKLM, Software, Microsoft, Windows Defender. In there I deleted both "DisableAntiSpyware" and "DisableAntiVirus".

My 4 Windows Defender Services are on Manual/Automatic. But Defender Antivirus Network Inspection Service and Windows Defender Antivirus Service are not running, but set to Manual and the Startup type is greyed out, so I cant change it and if I click on Start I get an error stating:

"Windows could not start the Windows Defender Antivirus Service service on Local Computer."

Error 577: Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.