Thanks. That was helpful -- until the next step.
Now I've edited that json file to accomodate for that new 6th element. And now Customize X-Ray brings it up without any errors. So, progress.
But on the Kobo device, I still get the same kinds of error as quoted above in koreader. And in nickel, there's a short delay as if something wants to pop up, but then nothing happens.
I also tried with a minimal xray-file, just a handful of entries made from Customize X-Ray, and there's the same problem.
I'm sorry, I don't want to bug you too much about this. After all, this worddumb-custom-x-ray.json method doesn't seem to be officially documented. But I just stumbled upon this WRT a book I was going to read, and then I got curious. And it does seem genuinely useful to have a method of curating the xray.
The non-json-file method seems to work, at least in koreader, not sure about nickel because I hardly ever use that. Is there still something I'm missing about that json file structure?