My problem is very similar to this one, although there are many other symptoms. I initially posted to that thread but decided to open a new thread.
I originally downloaded and installed Prep last week, but I don't recall what the exact version was. I was also running with a valid license when the trouble started. I am currently running 2018.1.2 (18.104.22.1686.1647) with a trial license. I switched to a trial license just to determine if there was something wrong with the validation of my license key. I am running Windows 7.
The original symptom was that Prep got stuck “thinking” when I clicked a node in the diagram, where the colored line in the lower pane would just cycle its animation but never finish, and nothing would load in the lower pane. Eventually I started seeing a lot of other symptoms like the 500 error referenced in the other post, red triangles on different parts of my diagram, etc. Opening my flow and just clicking Run All Flows brings up the popup with the timer, but it never really does anything. Sometimes I can close Prep, other times I have to kill it. This whole time I had changed nothing in the flow from when it had been working before; all I’d done was close prep, alter the raw files manually (see below), and re-open Prep.
I have tried a number of things, in the following order (to the best of my memory), to try and get it working again but nothing has worked. I tried using the “Repair” option in the setup. The I uninstalled and re-installed. It remembered my license key so I removed that and went with a trial license. Opened Prep fresh and tried dragging in one of my raw files from scratch. Also tried opening the sample World Indicators. Then I tried creating a simple CSV and importing it. Same results every time, it just thinks and eventually might throw errors after several minutes.
I have what I think is a pretty simple flow which brings in 2 text files, has a few calculations, a join, and 2 output files. Everything was working fine with my original files. I don't know whether what I did next is what caused the problem, or if it was coincidence. There's a field in each file which can be used to join them, I'll call it OrderKey, and there's basically an order header file and an order detail file. In my original data OrderKey matched completely between the files on an inner join, no mismatches. I altered the header file to contain a key value not in the detail, re-opened Prep and confirmed its report of mismatched data. Then I altered the detail file so that one of the key values in the header no longer appeared in the detail. That's when the symptoms started, but putting the original data back the way it started didn't fix things.
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.