How to fix New Outlook crashing on launch (0xc0000409)
New Outlook closes the moment you open it and logs a 0xc0000409 crash in Event Viewer. Corrupt local app folders are the usual cause, and rebuilding them fixes it. Jump to your situation below or work through the methods in order.
By Neeraj Singh ~6 min Updated Jun 2026 88% found this helpful
Error message
Outlook (new) closes immediately on launch. Event Viewer logs Exception code: 0xc0000409.
Summary
When the New Outlook app closes the instant you open it and Event Viewer records a crash with Exception code 0xc0000409, the cause is almost always corrupt local app folders, specifically the OLK and OneAuth folders under your local AppData. These hold cached app data and sign-in tokens, and when they are damaged New Outlook fails during start-up. The fix is to close Outlook and delete the OLK and OneAuth folders so the app rebuilds them clean on next launch. If that does not do it, Reset the app from Settings, make sure it is updated through the Microsoft Store, and as a fallback toggle back to classic Outlook to confirm the profile itself is fine. A full reinstall of New Outlook is the last resort.
What this error means
New Outlook keeps cached data and authentication tokens in local folders. The OLK folder holds app data, and OneAuth holds sign-in tokens. 0xc0000409 is a stack or heap corruption crash, and here it means the app hit damaged data in those folders while starting and bailed out.
Because the damage is in regenerable cache and token folders, deleting them is safe: New Outlook simply rebuilds them and you sign in again. That is why removing OLK and OneAuth is the primary fix, with reset, update and reinstall covering the rarer cases where the install itself is affected.
Common causes
The local OLK app-data folder is corrupt.
The OneAuth token folder is corrupt.
A damaged New Outlook installation.
An outdated build with a known start-up bug.
A broken cached sign-in state.
Expert insight
“New Outlook dying on launch with 0xc0000409 looks scary in Event Viewer, but it is almost always two corrupt folders, OLK and OneAuth, in your AppData. I close Outlook, delete both, and let the app rebuild them, you just sign in again afterwards. That clears the vast majority. If it still crashes, I Reset the app from Settings and make sure it is updated, but the folder delete is the fix nine times out of ten.”
Manager, Tech Support & Operations · 19+ years fixing Windows and system errors
✓ How to fix it
Method 1
Delete the OLK and OneAuth folders
1Close Outlook fully. Press Win+R, paste %localappdata%\Microsoft\ and press Enter.
2Delete the folders named OLK and OneAuth.
3Reopen New Outlook; it rebuilds them and prompts you to sign in.
Method 2
Reset New Outlook
1Open Settings, Apps, Installed apps, find Outlook (new), Advanced options, and choose Reset.
2This clears the app data and returns it to a clean state.
3Relaunch and sign in.
Method 3
Update New Outlook
1Open the Microsoft Store, go to Library, and update Outlook (new) to the latest build.
2An outdated build with a start-up bug can crash this way.
3Restart the app afterwards.
Method 4
Toggle back to classic to verify
1Switch the New Outlook toggle off to open classic Outlook.
2If classic opens fine, the mail profile is healthy and the issue is the New Outlook app data.
3Return to New Outlook after the folder fix.
Method 5
Reinstall New Outlook
1If it still crashes, reinstall Outlook (new) from the Microsoft Store.
2This replaces a damaged installation.
3Sign in again afterwards.
A 0xc0000409 crash on launch is corrupt local app folders, so close Outlook and delete the OLK and OneAuth folders under %localappdata%\Microsoft\, then reopen and sign in. If it still crashes, Reset the app from Settings and update it through the Store, with a reinstall as the last resort.
Frequently asked questions
Why does New Outlook crash on launch with 0xc0000409?
Its local OLK and OneAuth folders, which hold cached app data and sign-in tokens, are corrupt, so the app hits damaged data during start-up and crashes. Event Viewer logs it as exception 0xc0000409.
How do I fix it?
Close Outlook, press Win+R, open %localappdata%\Microsoft\, and delete the OLK and OneAuth folders. Reopen New Outlook; it rebuilds them and prompts you to sign in.
Is it safe to delete OLK and OneAuth?
Yes. They hold regenerable cached data and sign-in tokens, not your mail. New Outlook recreates them on next launch and you simply sign in again.
It still crashes after deleting the folders
Reset Outlook (new) from Settings, Apps, Installed apps, Advanced options, update it through the Microsoft Store, and as a last resort reinstall it.
Will I lose my email?
No. Your mail lives on the server, and classic Outlook uses a separate profile. Deleting the New Outlook cache folders does not remove messages.
How do I check it is this error?
Open Event Viewer, Windows Logs, Application, and look for an Application Error from olk.exe or Outlook with Exception code 0xc0000409 around the time it crashed.
Still not working?
If New Outlook still crashes after clearing the folders, resetting and reinstalling, a corrupt Windows user profile or a conflicting add-in can be involved; testing on a new local Windows account quickly isolates that. You can also submit your error to us for a tailored fix.
