The status page reads as below (I've copied the text here as it a) may not be accessible to everyone and b) often has old information removed / edited, c) this status page / information is usually pay-walled and only available once you've authenticated):
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Microsoft 365 Service health status
Title: Unable to access Microsoft 365 services
User impact: Affected users are unable to authenticate to and access Microsoft 365 services.
Current status: We've received reports of an issue affecting access to Microsoft 365 services. We've identified a degraded portion of infrastructure that manages authentication requests and have restarted that infrastructure to mitigate impact.
Scope of impact: This issue may potentially affect any of your users attempting to access Microsoft 365 services.
Start time: Tuesday, January 29, 2019, at 9:15 PM UTC
Preliminary root cause: A portion of infrastructure that manages authentication requests is degraded, affecting access to one or more Microsoft 365 services.
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Microsoft 365 Service health status
Title: Unable to access Microsoft 365 services
User impact: Affected users are unable to authenticate to and access Microsoft 365 services.
Current status: We've received reports of an issue affecting access to Microsoft 365 services. We've identified a degraded portion of infrastructure that manages authentication requests and have restarted that infrastructure to mitigate impact.
Scope of impact: This issue may potentially affect any of your users attempting to access Microsoft 365 services.
Start time: Tuesday, January 29, 2019, at 9:15 PM UTC
Preliminary root cause: A portion of infrastructure that manages authentication requests is degraded, affecting access to one or more Microsoft 365 services.