GAE flex app deployment failing with error code 400

I have been deploying a GAE flex app successfully over the last months.
I am now attempting to deploy the same application into another project that I have created as my staging environment.

The code is exactly the same, the app.yaml has only changed to reflect a new database and redis connections which I pass as environment variables.

The build goes well, but at the end I get the following:

"metadata": {
    "target": "apps/freesat-stage-project/services/default/versions/athiqtest4",
    "method": "google.appengine.v1.Versions.CreateVersion",
    "user": "[email protected]",
    "insertTime": "2017-11-08T11:11:59.089Z",
    "ephemeralMessage": "Deployment failed. Attempting to cleanup deployment artifacts.",
    "@type": "type.googleapis.com/google.appengine.v1.OperationMetadataV1"
},
"done": true,
"name": "apps/freesat-stage-project/operations/2e41e4e5-b9bd-42fb-9fd3-319ec322458b",
"error": {
    "message": "Deployment Manager operation failed, name: operation-1510139527034-55d76c211f490-1fcc927b-d3b3b590, error: [{"code":"RESOURCE_ERROR","location":"/deployments/aef-default-athiqtest4/resources/aef-default-athiqtest4-00ahs","message":"{\"ResourceType\":\"compute.v1.httpsHealthCheck\",\"ResourceErrorCode\":\"400\",\"ResourceErrorMessage\":{\"code\":400,\"errors\":[{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.checkIntervalSec': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"},{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.timeoutSec': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"},{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.unhealthyThreshold': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"},{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.healthyThreshold': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"}],\"message\":\"Invalid value for field 'resource.checkIntervalSec': '0'. Must be greater than or equal to 1\",\"statusMessage\":\"Bad Request\",\"requestPath\":\"https://www.googleapis.com/compute/v1/projects/freesat-stage-project/global/httpsHealthChecks\",\"httpMethod\":\"POST\"}}"}, {"code":"RESOURCE_ERROR","location":"/deployments/aef-default-athiqtest4/resources/aef-default-athiqtest4-hcs","message":"{\"ResourceType\":\"compute.v1.httpsHealthCheck\",\"ResourceErrorCode\":\"400\",\"ResourceErrorMessage\":{\"code\":400,\"errors\":[{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.checkIntervalSec': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"},{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.timeoutSec': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"},{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.unhealthyThreshold': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"},{\"domain\":\"global\",\"message\":\"Invalid value for field 'resource.healthyThreshold': '0'. Must be greater than or equal to 1\",\"reason\":\"invalid\"}],\"message\":\"Invalid value for field 'resource.checkIntervalSec': '0'. Must be greater than or equal to 1\",\"statusMessage\":\"Bad Request\",\"requestPath\":\"https://www.googleapis.com/compute/v1/projects/freesat-stage-project/global/httpsHealthChecks\",\"httpMethod\":\"POST\"}}"}]",
    "code": 13
}

My app.yaml looks as follow:

runtime: custom
env: flex

env_variables:
 REDIS_HOST: 10.154.0.3
 REDIS_PORT: 6379
 DB_NAME: freesat
 DB_USER: freesat-stage
 DB_PASS: password
 DB_HOST: /cloudsql/freesat-stage-project:europe-west2:freesat-stage
 GS_BUCKET_NAME: freesat-content
 DJANGO_CONF: conf.production
 JANRAIN_CLIENT_ID: clientid
 JANRAIN_SECRET: secret
 DOTMAILER_API_USER: [email protected]
 DOTMAILER_API_PASS: pass

health_check:
  enable_health_check: False

automatic_scaling:
  min_num_instances: 4
  max_num_instances: 15
  cpu_utilization:
    target_utilization: 0.4

beta_settings:
  cloud_sql_instances: 'freesat-stage-project:europe-west2:freesat-stage'

resources:
  cpu: 2
  memory_gb: 2

I cannot make sense of the error message. Any help would be appreicated.