Yesterday afternoon Pacific time (officially from 2:25pm PDT to 7:25PDT), Microsoft 365 users were unable to log in to multiple Microsoft 365 services that use Azure Active Directory, including Outlook, Microsoft Teams, Office.com, Power Platform, and Dynamics 365. Users who were currently logged in to services were not affected, and the services themselves continued to run, but anyone logged out was unable to log in during most or all of the 5 hour window.
Microsoft updated its Microsoft 365 admin center Service Health page last night to report a final status, that "(a)fter extended monitoring of our service, we have confirmed that the issue has been fully mitigated and is running within optimal service levels."
Microsoft is still investigating a root cause, with a post-incident report promised within 5 business days, but did post a preliminary cause, after initially ruling out a code change that was rolled back early in the outage, without success:
Preliminary root cause: A code issue caused a portion of our infrastructure to experience delays processing authentication requests, which prevented users from being able to access multiple M365 services.
We'll keep you posted on any further reports.