AWS Application Load Balancer (ALB) loosing requests

Question

My application allows uploading files with several simultaneous requests. Requests arrive at the CloudFront (CDN), go to the load balancer (ALB), and the load balancer passes them on to the API docker container, which is on the Elastic Container Service (ECS). Some requests complete successfully, others give 504 and 502 errors, intermittently.

When I change the CDN origin to go straight to the container, all requests work with fast response. So I think the problem is in the Application Load Balancer. I already increased the idle timeout in the ALB, but the problem remains the same. I also tried increasing the Origin Connection Timeout in CloudFront, but it didn't work either.

I have only 1 Docker container in the service.

Can someone help me on this issue? Best regards! Fernando