Resource handler returned message: ECS Deployment Circuit Breaker was triggered (HandlerErrorCode: GeneralServiceException)

Goal
I'm trying to run a service using a task definition which is a flask server image for backend.

Problem

• I keep getting this error message - There was an error deploying the backend-flask container. Resource handler returned message: "Error occurred during operation 'ECS Deployment Circuit Breaker was triggered'." (RequestToken: ..., HandlerErrorCode: GeneralServiceException)

I tried

• Removed the cluster and service. Create a cluster, then create a service using the task definition (provided below) from scratch all over again.
• Attached the AmazonEC2ContainerRegistryReadOnly policy to both Task role and Execution role to make sure that the service can access the ECR for the flask image.
• Updated the flask Dockerfile (from EXPOSE ${PORT} to EXPOSE 4567)
• Checked the CloudFormation log but the error message is too general, unable to pinpoint where is wrong (please see the attached image).

Can someone please help me:

1. how to reason/identify the cause based on the clues I suggested here.
2. where to look and what to fix.

I appreciate your taking time for my trouble. Thank you.

CloudFormation log

Task definition: backend-flask.json

{
    "family": "backend-flask",
    "executionRoleArn": "arn:aws:iam::<ACCOUNT_ID>:role/appServiceExecutionRole",
    "taskRoleArn": "arn:aws:iam::<ACCOUNT_ID>:role/appTaskRole",
    "networkMode": "awsvpc",
    "cpu": "256",
    "memory": "512",
    "requiresCompatibilities": [ 
      "FARGATE" 
    ],
    "containerDefinitions": [
      {
        "name": "backend-flask",
        "image": "<ACCOUNT_ID>.dkr.ecr.<AWS_REGION>.amazonaws.com/backend-flask",
        "essential": true,
        "healthCheck": {
          "command": [
            "CMD-SHELL",
            "python /backend-flask/bin/flask/health-check"
          ],
          "interval": 30,
          "timeout": 5,
          "retries": 3,
          "startPeriod": 60
        },
        "portMappings": [
          {
            "name": "backend-flask",
            "containerPort": 4567,
            "protocol": "tcp", 
            "appProtocol": "http"
          }
        ],
        "logConfiguration": {
          "logDriver": "awslogs",
          "options": {
              "awslogs-group": "cruddur-week6-ecr",
              "awslogs-region": "us-east-1",
              "awslogs-stream-prefix": "backend-flask"
          }
        },
        "environment": [ ...  ],

        ]
      }
    ]
  }