This error essentially means that a child Relay or Client requested a site that is not currently subscribed in the environment (or at least it’s masthead is not in the Relay’s actionsite). Generally, if the site in question is not subscribed in the Console, this error can safely be ignored (we are exploring methods to better handle these sorts of errors moving forward).
Isn’t the source of the issue that an Endpoint is requesting the missing site content from the Relay, and the Relay is unable to retrieve it from the server?