What version of Tableau Prep are you leveraging? I would make sure you have upgraded to the latest version. I have seen some reports of this error, but the users were all using an early version of Tableau Prep 2018.2.1, the Tableau Dev team is constantly improving the product and I would suggest using the latest release.
Another troubleshooting step would be to try recreating the flow and see if the error occurs again.
Hope this helps!