Outlook app won't start, it just shows an error message (error 440)

Question

Online version works fine, but the app is unusable. I believe this problem started after I was forced to update to the new outlook

Answer 1

Hi @Brie Wilga, 

Welcome to Microsoft Q&A forum. 

Thank you for reaching out and sharing your concern. I can totally understand how disruptive it is when Outlook on the web works but the desktop app will not launch. This behavior is commonly caused by corrupted local New Outlook data or sign in state. When those local files break, New Outlook can fail during startup even though your mailbox is fine online.  

To resolve the issue, please try these steps 

1. Close New Outlook completely 

• Open Task Manager, select Microsoft Outlook, then choose End task. Reopen New Outlook. 

 

2. Clear New Outlook cache 

• Press Windows + R  

 

• Open %localappdata%\Microsoft\olk, then delete UserSettings.json.  

 

• If the issue continues, open %localappdata%\Microsoft and delete the Olk folder and the OneAuth folder.  

3. Reinstall New Outlook 

Uninstall New Outlook from device Settings 

• Select Start, then open Settings. 
• Select Apps, then Installed apps. 
• Select the app, then select More > Uninstall and confirm. 

Reinstall New Outlook:

• Once Outlook is uninstalled, open the Microsoft Store app using search. 
• In the Microsoft Store app, search for Outlook for Windows and then Install. 

• Once the Outlook app is installed, launch the app and add your email accounts one by one. 

I hope this information is helpful. Please follow the steps above and let me know if it works for you. If not, we can continue working together to resolve the issue.  

Thank you for your patience and understanding. If you have any questions or need additional assistance, please don’t hesitate to reach out so I can continue to support you. If you found the response useful, please consider marking it as accepted, as this may help other community members who are looking for similar guidance.  

I look forward to continuing the conversation. 

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