We have magento running on AWS Fargate behind a load balancer. It works great when it is not scaled(i.e only one task is running), however when it scales to 2 or more tasks I am not able to login as a customer or administrator and we aren't getting any login errors, it just redirects to itself. I can only login successfully when only one task is running. We're running magento 2.3.2 on AWS Fargate behind a load balancer. Any ideas why this might be happening?
You should use Amazon Elasticache service to store user session in the case. Also please pay attention that you also need RDS and EFS services for container based Magento setup
See more details here https://aws.amazon.com/ru/quickstart/architecture/magento/