Update: the checkout flow is back online. We have migrated merchant login, product catalog, and checkout services to replacement databases brought up from point-in-time backups, and these services are now operational. Most merchants should see checkout behaviour returning to normal.
The remaining background services (settlements, loyalty, communications) are being brought online over the next few minutes.
Our cloud provider's engineering team is continuing to investigate the original upgrade state separately; that work is no longer on the customer-facing path.
We're keeping this incident open while we monitor for stability. We will post a final resolution update once we have confirmed clean behaviour across all services.