HTTP response status code 561 is an unofficial server error that is specific to the AWS Elastic Load Balancer (561 Unauthorized) and Pantheon (561 No site detected). In the case of AWS, it is an HTTP Authentication failure, whereas, with Pantheon, it indicates that there is a problem with the site.
Usage specific to the AWS Elastic Load Balancer
When the 561 Unauthorized status code is received, it means that a listener rule has been configured to authenticate users but the identity provider (IdP) returned an error code during the Authentication process.
Usage specific to Pantheon
When the 561 No site detected status code is received, it means that a site has been created but no content management system (CMS) has been installed. It can also occur instead of Pantheon HTTP error 403 Forbidden (Directory Listing Denied) if there is no index file present.
The 561 Unauthorized / No site detected status code is a server error that is specific to AWS Elastic Load Balancer, where the HTTP request can not be completed due to an authentication failure, and to Pantheon, when the site has not been properly configured.