なんたらFridayですね。一部テナント（？）でサインイン周りの障害があったようです。（今は解決済み）
とりあえず引用（パーマリンクがないので）
Azure Active Directory – Authentication Errors – Resolved
Summary of impact: Between 08:30 and 11:30 UTC on 06 Apr 2018, a subset of Azure Active Directory customers in East Asia and Europe may have experienced difficulties when attempting to authenticate into resources which are dependent on Azure Active Directory. Downstream impact was reported by a number of Azure services for which customers may have experienced: Backup: failures for the registration of new containers and backup/restore operations. Storsimple: new device registration failures and Storsimple management/communication failures. Azure Bot Service: bots reporting as unresponsive. Visual Studio Teams Services: higher execution times and failures while getting AAD tokens in multiple regions. Media Services: authentication failures. Azure Site Recovery: new registrations and VM replications may also have failed.
Preliminary root cause: Engineers determined that instances of a backend service responsible for processing authentication requests became unhealthy preventing requests from completing.
Mitigation: Engineers performed a recovery of the impacted backend service .
Next steps: Engineers will continue to investigate to establish the full root cause and a full root cause analysis will be available in 72 hours.
主にEast Asia、EuropeにあるAzure ADの顧客が影響を受けたようです。Azure AD認証を使う一部のサービス（Media Serviceやバックアップ、Bot、VSTS、Office 365とか）で認証が失敗するなどの状況だったようす。原因は認証要求処理を担当するバックエンドのインスタンスが不健全になったかららしい。
