AWS ECS agent won't start

I found the answer myself. The checkpoint file is located in this directory: /var/lib/ecs/data/ecs_agent_data.json. Just delete that json file and start the ECS agent again running start ecs. Thanks to this forum post, which indirectly led me to the answer: https://forums.aws.amazon.com/thread.jspa?messageID=625643

delete checkpoint file form below location

sudo rm /var/lib/ecs/data/agent.db

now start ecs agent

sudo systemctl start ecs

sudo systemctl status ecs