Clueless on that one, sorry
Also- if the same extract is scheduled using the Tableau scheduler it runs for 38 minutes and finishes without error.
Did this ever get resolved? We've been suffering from this same exact error and it's quite troublesome.
Just came across this old thread while trying to track down a "502 Proxy Error" message from tabcmd.
It turns out that the same error message can be triggered by various different underlying issues - it basically indicates that the connection between two of Tableau's server processes has gone away. In my cases this was because one of the server processes had failed with an Out of Memory error. Other cases I've seen on the forums and an article in the knowledgebase indicate that it can be due to exceeding a timeout value. Just pointing out that raising the timeout value isn't necessarily the answer - though if the failure happens after 30 minutes (or whatever your timeouts are set to) that would be the place to start.