I just hit this issue too.

Already using Lets Encrypt i another setup, but thought using DO load balancer would ease management. Our webpage alone has 5 domains - running a load balancers for each domain is a weird restriction.

Your load balancer checks the health of its registered instances using either the default health check configuration provided by Elastic Load Balancing or a custom health check configuration that you specify. The health check configuration contains information such as the protocol, ping port, ping path, response timeout, and health check interval. An instance is considered healthy if it returns a 200 response code within the health check interval.

If the current state of some or all your instances is OutOfService and the description field displays the message that the Instance has failed at least the Unhealthy Threshold number of health checks consecutively, the instances have failed the load balancer health check. The following are the issues to look for, the potential causes, and the steps you can take to resolve the issues.

Issues

Health check target page error
Connection to the instances has timed out
Public key authentication is failing
Instance is not receiving traffic from the load balancer
Ports on instance are not open
Instances in an Auto Scaling group are failing the ELB health check