A bug in the deployment of an Azure AD service update caused Monday’s Office 365 outage. People worldwide were unable to login to Office 365 and other related services, including Microsoft Teams, Office.com, Power Platform, and Dynamics365. A service update mistakenly hit the production environment and caused service availability to degrade. A final report from Microsoft should become available by the end-of-day on October 3rd. The Americas and Asia-Pacific regions suffered the most problems when logging into services.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-explains-the-cause-of-the-recent-office-365-outage/