We've determined that an authentication certificate has expired causing, users to have issues using the service. We're developing a fix to apply a new certificate to the service which will remediate impact. Further updates can be found under TM202916 in the admin center. — Microsoft 365 Status (@MSFT365Status) February 3, 2020

It's odd that Microsoft would forget to renew a critical authentication certificate, and it was rude timing, as so many were attempting to log in and start their week.

Almost three years old, Microsoft Teams is the company's Slack competitor. Last year, it added new features like private channels and custom video chat backgrounds, and it recently launched its first TV ad campaign. Needless to say, today's outage probably doesn't help in Microsoft's efforts to take on Slack.