MO1420954: Microsoft 365 suite Service Health Incident (False Positive)

[365 Service Alert]

Users are unable to access Microsoft Copilot web and m365.cloud.microsoft.                                                                                                                                                                                                                                  

Users are unable to access Microsoft Copilot web and m365.cloud.microsoft ID: MO1420954 Issue type: Incident Status False Positive Impacted services Microsoft 365 suite Details Title: Users are unable to access Microsoft Copilot web and m365.cloud.microsoft User impact: Users were intermittently unable to access Copilot web and other pages that redirect to the m365.cloud.microsoft URL. More info: Access eventually timed out with an error message stating, "Bad Request - Header Field Too Long. HTTP Error 400. A request field is too long." The impacted URL was m365.cloud.microsoft and other URLs which redirect to this page, including but not limited to: - office.com - portal.microsoft.com Users may have been using URLs such as office.com and the navigation Waffle to access M365 applications in the browser, such as Outlook. For this scenario, when the URL redirected to m365.cloud.microsoft they may have experienced impact, however, users could access M365 applications in the browser directly through the product’s specific URL (such as: outlook.office.com). As a potential workaround, users could have used the desktop version of the Microsoft 365 applications. Additionally, users may have experienced temporary relief by refreshing the cache when trying to access through the browser. Final status: We completed the traffic rerouting and have confirmed with previously impacted users that impact is now remediated. Scope of impact: This issue could have impacted any user attempting to access Copilot web, or a URL which redirects to m365.cloud.microsoft. Start time: Tuesday, July 7, 2026, at 5:00 PM UTC End time: Friday, July 10, 2026, at 6:00 PM UTC Preliminary root cause: A recent change which modified the route that back-end service calls take when users access the affected apps unexpectedly introduced impact. Next steps: - We're continuing to investigate why the offending change introduced impact and why it wasn't detected prior to deployment. We'll provide a preliminary Post-Incident Report within two business days and a final Post-Incident Report within five business days. Is this message helpful? Did you find this email helpful? Yes No To customize what’s included in this email, who gets it, or to unsubscribe, set your Service health preferences. If you are receiving this email because your Admin added you as a recipient, please contact your Admin to unsubscribe. Privacy Statement Microsoft Corporation, One Microsoft Way, ​Redmond, WA 98052​