Microsoft has resolved an ongoing incident that blocked access to some Defender XDR portal capabilities, including threat hunting alerts. The issue was caused by high CPU utilization due to increased traffic. Mitigation measures were applied, and availability has recovered for some impacted customers. Microsoft is now analyzing HTTP Archive traces and expects to release preliminary and final post-incident reports within two and five business days, respectively.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-defender-portal-outage-blocks-access-to-security-alerts