Sorry to hear that you're having issues. From the stack trace, it looks like something licensing related, which is curious. But whether it's a one-off or a repeated issue, it's still a bug. Can you open a support case for this? That's the best way for us to collect the information needed to suss this out.
Thanks for responding Isaac, will surely open a case.
I just started experiencing the same thing in Tableau Prep, but with text files. I experienced other strange symptoms before this happened. Tried to do repair of the install, but it's still acting up. I uninstalled it re-installed. Things seemed okay at first, then it started acting up again. I even opened the sample World Indicators (which I had never previously even looked at). When I click the first step in my flow, the data never loads in the lower pane, the blue line just keeps moving. Not good timing, as I am trying to support a paying client… guess I’ll have to go back to the wonky Excel I was using before I tried to use the new Prep… and I don’t feel comfortable asking them to get a new paid Tableau license so they can run these themselves either.