These endpoints are served by the Amazon Elastic Load Balancers. As a result, the actual endpoints will vary dynamically across Amazon's IP ranges.
The agent will perform a lookup to the applicable DNS entry, which may resolve to one of multiple Amazon Web Services based subnets. As a result, if your firewall does not support outbound filtering by domain name, then you may need to open all outbound traffic to 443/tcp to accommodate this service.
Additionally, verify that your proxy server can externally communicate.