You don't by any chance have the hyper open in tableau desktop at the same time do you? As I have had this issue when trying to run a flow when I had the .hyper open in Tableau desktop.
Tableau 2018.3 is now out. Perhaps try updating the software first and see if this fixes your problem.
Thanks Rory and Catherine,
I didn't have the extract open (mainly because it always failed to run!) and it also failed in Tableau Prep version 3. I spoke to support and it seems like it's going to be a bug. I'm at the Tableau Conference now and will ask around.