On Windows Server 2019, I wanted to switch off Windows Defender Anti-Virus realtime scanning to test something. I went to "Windows Security" and then cliecked on the "Open Windows Security" button. This brought up the following error message:

You need a new app to open this windowsdefender

After some searching, I found that you need to install the missing app yourself, it seems to be a bug in Windows Server 2019. To do this, open Powershell as Administrator and type (all in one line):

Add-AppxPackage -Register -DisableDevelopmentMode "C:\Windows\SystemApps\Microsoft.Windows.SecHealthUI_cw5n1h2txyewy\AppXManifest.xml"

That's it. Now you can get the the Windows Defender settings right away (no server restart needed).