Microsoft Exchange admins who attempted to access the admin portal at admin.exchange.com suddenly found that their browsers were issuing warnings that the connection was not private due to an expired SSL certificate. Depending on the browser, users are blocked from accessing the site as a security precaution or shown an alert that the data may not be secure. The certificate expired today, Sun, 23 May 2021, at 12:00:00 UTC. Microsoft states that they are aware of the issue and are working on a fix.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-exchange-admin-portal-blocked-by-expired-ssl-certificate/