URL issue with AWS load balancer and SSL certificate
You should look at log the load balancer. the 503 might be generated there (ie. the LB can't reach the backend) or it might be that the LB can reach the real servers hosting civi, but they can't process the request

Assuming you trust amazon (a big assumption), you might want to say that the https LB can talk to an unsecure civi server (via http).

Otherwise if you want to be more secure and don't trust amazon, you might need to configure your civicrm server so they handle https too.