If successful, this indicates there is an issue maintaining a network connection between the client computers and the Tableau Server.
When downloading via "localhost" or the IP 127.0.0.1, the request comes from the computer running Tableau Server and connects directly to the data without ever leaving the computer's environment. Successfully downloading in this scenario proves there is nothing wrong with the data (it is not corrupted) or the Tableau Server (not a permissions or configuration issue).
When downloading via "tableau.mycompany.com" the request comes from the client computer, goes to a network device (router, firewall, switch, etc.) and then connects to the Tableau Server. When attempting to download, that network device is saying the connection is taking too long and cuts it off after reaching the timeout, throwing the HTTP 504 error. Other datasources may not be affected since they are downloaded before hitting the timeout limit.