I experience slowness when accessing a Tableau extract (about 80 million rows) and sometimes Tableau will throw out an error.
"<h2>502 - Web server received an invalid response while acting as a gateway or proxy server.</h2> "
"<h3>There is a problem with the page you are looking for, and it cannot be displayed. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server.</h3> "
At the back end, the Tableau logs shows repeatedly a data engine error 306 message.
Has anyone encountered an error like that and did you manage to resolve your problem?