Basically the error means that no Application in your EC2 instance is listening on port 8888. Did you start Jupyter notebook inside the Remote machine?
Try the command jupyter notebok list, if there’s jupyter server running in EC2, you will see it, otherwise it will come up empty. If it’s empty run Jupyter Notebook.
Because you are using Tunneling of ports, if for any reason you loose the SSH connection, your notebook is not accessible. It might be better to open up port 8888 on the server by modifying the Security Groups (on EC2) and then directly connect via http://<ec2-servername>:8888