Outlook Error Codes

How to Fix Outlook Web Error 401 When Signing In

Maira Mehtab Sep 22, 2026 868 views

Question details

The user is unable to access Outlook Web due to a persistent HTTP 401 error that occurs across multiple browsers.

Product: Outlook Web
Device & OS: not provided
Scenario: Attempting to sign in to the Outlook Web application.
Observed behavior: Outlook Web displays an HTTP 401 error accompanied by "BootResult: fail" and "ServerError" messages, which persists even after completely clearing browser cache and data.

Before you start

Verify if there are any ongoing Microsoft 365 service outages by checking the Microsoft 365 Status account on social media or asking colleagues if they are experiencing similar login issues.

Solution 1Recommended

Wait for Microsoft Backend Service Restoration

This specific error combination typically indicates a backend authentication problem on Microsoft's servers rather than a local client issue.

Because multiple users generally report this specific Error 401 (BootResult: fail) simultaneously, it is almost always tied to temporary backend service degradation or maintenance. Local troubleshooting like clearing cache will not bypass a server-side authentication failure.

Monitor for updates

Check external status trackers or the official Microsoft 365 Status updates to confirm if there is a known ongoing issue with Exchange Online or Outlook Web.

Retry sign-in later

Wait a few hours for Microsoft engineers to restore the authentication services, then attempt to log in to your Outlook Web account again.

No local action needed: Since clearing browser data and switching browsers do not affect backend errors, you can save time by simply waiting for the server connection to be restored.

Solution 2

Contact Your IT Administrator or Microsoft Support

If the error persists after a significant amount of time and is not part of a widespread outage, professional administrative support is required.

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Frequently Asked Questions

What does HTTP Error 401 mean in Outlook Web?

HTTP 401 is an "Unauthorized" error. In the context of Outlook Web with a "BootResult: fail" flag, it means Microsoft's servers are temporarily unable to validate your login credentials due to a backend synchronization or server health issue.

Will clearing my browser cache fix the 401 ServerError?

No. While clearing browser cache fixes many web issues, a 401 error accompanied by "ServerError" and "BootResult: fail" is a server-side problem. Local troubleshooting will not resolve it.

How can I check if Microsoft 365 services are down?

Administrators can check the Service Health dashboard inside the Microsoft 365 Admin Center. Standard users can monitor the official @MSFT365Status account on X (formerly Twitter) or check popular downtime tracking websites.

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