We'll close this incident now with Microsoft's mitigation in place. Future updates and postmortem can be gathered from MO305726 in the Microsoft 365 Admin Centre.
Posted Dec 16, 2021 - 14:13 AEDT
Monitoring
Microsoft have reported that they've completed mitigation efforts and are validating that the impact has been mitigated.
Posted Dec 16, 2021 - 13:57 AEDT
Update
Users with Microsoft 365 Admin access may want to follow issue MO305726 in the Service health portal. We'll aim to update this page as well once a resolution is identified.
Microsoft is halting a production rollout and attempting to identify the underlying issue. You may still receive intermittent 503 errors. Please try again shortly.