5/13/2020, 1:00 PM, RESOLUTION: Microsoft has completed rolling back the change that caused the web app login issue. All users should now be able to sign into the Outlook web app successfully.
5/13/2020, 11:35 AM, UPDATE: Microsoft has identified the cause and is rolling back a recent change to restore service for the Outlook Web app. No ETA on when this solution will be complete, but some users may be able to access the Web app before others.
*****Original Message Follows*****
5/13/2020, 11:00 AM: Some people are encountering an error when they try to access their email or calendar via Outlook in a web browser. If you encounter an error, you may still be able to access your mail and calendar from the Outlook desktop application for Windows and macOS, or from the mobile Outlook app for iOS and Android. Staff are diagnosing the error for the web app.