I have a Flask application running on ECS behind an application load balancer. My website successfully delivers content over HTTP. It does not deliver content over HTTPS. All of those requests result in 502 Bad Gateway errors.

• The ECS cluster passes health checks.
• My health checks leverage port 443.
• My ALB has two listeners: one that redirects from 80 to 443, and one that uses HTTPS protocol over port 443 and is linked to an AWS ACM certificate.
• My AWS ACM certificate is DNS-verified for the domain.
• My security groups attached to the load balancer listeners allow inbound traffic from the internet on port 80 and 443.
• My ECS cluster itself has security groups that allow it to make outbound requests on 443.
• My application load balancer logs show request_processing_time, target_processing_time, and response_processing_time of -1.
• My application load balancer logs show an elb_status_code of 502, and a target_status_code of -. Per AWS, this indicates a load balancer issue.

Why do all requests to my website result in 502 Bad Gateway errors?