Update: September 28, 2020, 22:45 PT
The issue has been resolved as of September 28, 2020, 22:45 PT.
September 28, 2020, 16:00 PT
Due to an issue with Microsoft’s service, some users may be unable to access multiple Microsoft services, including Microsoft Teams. Existing sessions are not impacted and any user who is logged in to an existing session would be able to continue their sessions.
Microsoft is investigating this issue and updating their status page here: https://status.office365.com/