Members' portal returning 500 errors since last night: “password authentication failed”

Mutuelle Tadamoun (a fictional supplementary health insurer, 140,000 members) offers an online portal: reimbursements, certificates, hospital cover approvals. The portail-adherents application runs on ECS Fargate (cluster prod-tadamoun, 3 tasks) and uses an RDS PostgreSQL database.

The application user's password is stored in Secrets Manager (prod/adherents/db), with automatic rotation every 30 days enabled two months ago by the security team.

Since 02:31, the portal has been returning 500 errors. Branches open at 08:30 and partner clinics are waiting for cover approvals. Nobody has deployed since 12 October.

You are on your workstation with the AWS CLI v2 (region eu-west-3).