Windows 11 - Windows Defender will not open

 After upgrading to Windows 11, Windows Defender can stop opening and may give the below error

To fix this open PowerShell as admin and type "Get-AppxPackage Microsoft.SecHealthUI -AllUsers | Reset-AppxPackage"

Using 3rd party antivirus on Windows server 2016 and 2019

If you using a 3rd party AV on Windows server 2016 or 2019 the Windows Defender Antivirus is locked in to active mode regardless of set up.

https://docs.microsoft.com/en-us/windows/security/threat-protection/microsoft-defender-antivirus/microsoft-defender-antivirus-compatibility

The options are to use the registry key to switch it to passive mode

• Path: HKLM\SOFTWARE\Policies\Microsoft\Windows Advanced Threat Protection
• Name: ForceDefenderPassiveMode
• Type: REG_DWORD
• Value: 1

Or uninstall which can be done via PowerShell

• Uninstall-WindowsFeature -Name Windows-Defender
• Uninstall-WindowsFeature -Name Windows-Defender-GUI

https://docs.microsoft.com/en-us/windows/security/threat-protection/microsoft-defender-antivirus/microsoft-defender-antivirus-on-windows-server-2016#need-to-uninstall-microsoft-defender-antivirus