After configuring a ECS cluster named docker-cluster1 in AWS and also configuring the amazon-ecs-plugin to use it, I get this error in the jenkins.log:

INFO: Started provisioning ECS Slave docker from aws-ecs with 1 executors. Remaining excess workload: 0
Sep 26, 2017 12:32:48 PM hudson.slaves.NodeProvisioner$2 run
WARNING: Unexpected exception encountered while provisioning agent ECS Slave docker
com.amazonaws.services.ecs.model.ClusterNotFoundException: Cluster not found. (Service: AmazonECS; Status Code: 400; Error Code: ClusterNotFoundException; Request ID: f7bc84e2-a2a5-11e7-801f-1bbc4489a697)

The error makes no sense as the plugin does autodetect the cluster and fills it in the dropdown in the Manage Jenkins interface (so there's no manual input error possible...).