It happened I think three times, all in one fight. in the Dry River area right near the start at the first bug hives. I was hacking and slashing away, and I noticed seeing “TEST” a few times. It was only that fight though; I was looking for it in later fights to get you a screen shot and no bueno.
My char is a BD with lots of dodge stuffs, which might be where the problem originated, I’m not sure. I don’t think mechanically anything seemed like a problem.
Edit: for skills, I started with shift, smoke bomb and decoy, launched lethal mirage and I don’t think the problem started here, then I went into dancing strikes, and this is when I saw the “TEST” come up. I think.
The file is specifically called player.log… you shouldnt be looking for a json file… The .log file is a plain text file that can be uploaded to this thread (upload icon). This file is not with the game installation - its in a different place with your savegames etc.
(from the link LLama provided above, you will see that its in various folders depending on your OS)…
if you cannot find an Appdata folder in your File Explorer, then the folder is most likely hidden by Windows and you need to unhide it (again, llamas next google link shows you how to do that).