Hi Stefan , please do not persist in trying to run that first powershell command as advised by the advisor. That command does not work in Windows 11.
As you have discovered the path C:\Windows\SystemApps\Microsoft.Windows.SecHealthUI_cw5n1h2txyewy\AppXManifest.xml
does not exist.
Try the following command from an admin Powershell:
Get-AppxPackage *SecHealthUI* -allusers | % {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppxManifest.xml"}
If that doesn't work, you may need to run a repair installation.
Download the ISO at the link below:
https://www.microsoft.com/software-download/windows11
To start the repair, right click on the ISO image and select Mount
Then run Setup.exe