amazon-ecs-agent is always restarting

That message indicates your instance cannot do an HTTPS POST to the ecs.eu-central-1.amazonaws.com endpoint.

Make sure your security groups and/or VPC configuration allows outbound HTTPS to your ECS endpoint.

You can confirm endpoint connectivity from the instance to the endpoint the command line:

curl https://ecs.eu-central-1.amazonaws.com

That should return a message similar to the following:

<MissingAuthenticationTokenException>
  <Message>Missing Authentication Token</Message>
</MissingAuthenticationTokenException>