Fix New Outlook Error 0xc0000409 on Windows 11

The new Outlook app crashes immediately on launch with error code 0xc0000409. This happens when the Outlook identity cache or authentication folders get corrupted, usually after an update or profile sync issue.

The Fix

Close Outlook completely, then delete these folders:

taskkill /f /im olk.exe

Remove-Item -Path "$env:LOCALAPPDATA\Microsoft\Olk" -Recurse -Force -ErrorAction SilentlyContinue

Remove-Item -Path "$env:LOCALAPPDATA\Microsoft\OneAuth" -Recurse -Force -ErrorAction SilentlyContinue

Remove-Item -Path "$env:LOCALAPPDATA\Microsoft\IdentityCache" -Recurse -Force -ErrorAction SilentlyContinue

Restart Outlook. It will rebuild these folders on launch and prompt you to sign in again.

If That Doesn't Work

Reset the entire Outlook app through Windows Settings:

Get-AppxPackage OutlookForWindows | Reset-AppxPackage

You'll need to reconfigure your accounts after the reset.

The 0xc0000409 error is a generic application crash code. In Outlook's case, it usually points to corrupted authentication state rather than a deeper Windows problem.