HTTPAPI ALB integration over VPCLink to TargetGroup return 500 error

Hi, Here is my configuration

mydomain.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 500 errors https://mydomain.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. As far I see requests are not hitting the Fargate Service but I am not sure.

Sample access log from API GW HTTP API:

{
    "requestId": "some_req_id=",
    "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/107.0.0.0 Safari/537.36",
    "sourceIp": "176.232.**.**",
    "requestTime": "01/Nov/2022:09:25:37 +0000",
    "requestTimeEpoch": "1667294737",
    "httpMethod": "GET",
    "path": "/otherPath",
    "status": "500",
    "protocol": "HTTP/1.1",
    "responseLength": "35",
    "domainName": "mydomain.com",
    "error_Message": "Internal Server Error",
    "integrationErrorMessage": "-",
    "integration_Error": "-",
    "integrationStatus": "200",
    "integration_Status": "-",
    "integration_IntegrationStatus": "200",
    "integrationLatency": "5"
}

What am I missing? Why is it sooooo hard to find what is causing the error? I think configuration is fine but somehow it is really hard to make it work. Unbelievable!