Microsoft is working on resolving an issue that prevents users from accessing the Microsoft 365 Admin Portal. The issue is also affecting several other Office 365 apps and services. Microsoft says that a recently-applied update seems to be behind this ongoing issue. Users have also reported [1, 2, 3] that this page also keeps going down. Microsoft Teams users also experienced issues joining meetings hosted in North America last week, with any user attempting to join such Teams meetings potentially being impacted by the issue.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-365-admin-portal-is-down-office-365-services-also-affected/