The "exec format error" when running docker containers on AWS ECS is caused by a mismatch between the host machine's architecture and the docker image's architecture.

For example, if you are launching an ECS task on an EC2 instance with an ARM-based processor like Graviton, but your docker image is built for x86, you will see this error. The executable in the docker image is incompatible with the host architecture.

To fix this:

• Check if your ECS cluster is running on EC2 instances with ARM processors (like Graviton). If so, you need to build your docker image specifically for ARM.

• Make sure the docker image you are using matches the host architecture. For ARM, use images tagged with "arm64" or push your own image built on ARM.

• If running multiple architectures, you can use ECS Anywhere to build multi-architecture images to run on heterogenous clusters.

• Double check the Dockerfile for the image to ensure it can support multiple architectures if needed.

With matching architectures between the docker image and ECS host, the "exec format error" should no longer occur when launching ECS tasks. Carefully checking the host setup and docker image builds will avoid this mismatch.