HTTP API PROXY INTEGRATION_NETWORK_FAILURE Error

Hi,

Here is my setup: my.domain.com -> API GW Custom Domain -> HTTPAPI -> Route (/api/{+proxy}) -> VPCLink -> ALB -> HTTPS Listener -> TargetGroup (Type: Instance) -> ECS Fargate Service

HTTPAPI integration has the following parameter mapping: path -> overwrite -> /$request.path.proxy (I want to get rid of "api" part in the url)

when I make below request I got 503 errors time to time: https://my.domain.com/api/otherPath

I have enabled access logs on HTTPAPI but they show very limited information. ALB logs are sent to S3 bucket so it is almost impossible to track request.

Why do I get 503 errors ?

Access log from HTTP API:

{
    "requestId": "req_id=",
    "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0.3 Safari/605.1.15",
    "sourceIp": "**.**.**.**",
    "requestTime": "09/Nov/2022:18:07:24 +0000",
    "requestTimeEpoch": "1668017244",
    "httpMethod": "GET",
    "path": "/otherPath",
    "status": "503",
    "protocol": "HTTP/1.1",
    "responseLength": "33",
    "domainName": "my.domain.com",
    "basePathMatched": "(none)",
    "error_Message": "Service Unavailable",
    "error_responseType": "INTEGRATION_NETWORK_FAILURE",
    "integrationErrorMessage": "-",
    "integration_Error": "-",
    "integrationStatus": "200",
    "integration_Status": "-",
    "integration_IntegrationStatus": "200",
    "integrationLatency": "9002"
}

Please help, Thanks