The instance should not be in the ELB security group.

playbooks/edx-east/edx_service.yml

@@ -307,9 +307,7 @@
         region: "{{ aws_region }}"
         wait: "yes"
         group_id:
-          #Apply the ELB security group to the instances so they can talk to teach other
           - "{{ service_sec_group.group_id }}"
-          - "{{ elb_sec_group.group_id }}"
         key_name: "{{ service_config.key_name }}"
         vpc_subnet_id: "{{ created_service_subnets.results[item | int % created_service_subnets.results | length].subnet_id }}"
         instance_type: "{{ service_config.instance_type }}"