Which version are you using? They just released a new version a couple days ago. If you haven't upgraded, then try upgrading to see if that issue has been corrected. If it still doesn't work, I think you may need to open a case with Tableau Support.
Thanks for the question! I'm sorry you are running into these issues. As Ken asked, it would be great to know what version of Tableau Prep you are using. Could you log a support case when you get a chance?
I was using version 2018.1.1, but I just upgraded to the latest version on the web site (1.2). It looks like the same problem is occurring, but let me try again, and if it hangs/whites out the screen, I'll submit a case.
The workaround is to connect to the data source again and set the sample to Random, then I disconnect the workflow from the original data source and connect it to the new one.
FYI, I tested again under 1.2, and it still happens. Another workaround - if I disconnect my workflow from the data source, then change the source to use random sampling, then reconnect the data source to the flow, everything works. Changing the sampling type for the large workflow results in a hang, and after a few minutes, the screen goes white. I can still click on menus, but nothing happens other than seeing the dropdown.
I'll submit a case.
I'm glad you found a workaround, Jeff! Thanks for submitting a case, it's important that we track these issues.