There are a couple of options outlined in the article below that may help.
Can you please post back to let us know how you get on?
Work with your proxy administrator to increase the proxy timeout interval.
Revise the views so that the total load time on Tableau Server is lower than the proxy timeout.
The proxy is timing out before the HTTP request for the view returns a response. The appearance of this error indicates that the view is taking at least as long to render as the proxy timeout interval.