I mentioned in the other thread a change in firmware 4.20 to add some local storage. This apparently stems from
https://github.com/kobolabs/Kobo-Reader/issues/59. There is a link in the original report for a page that uses local storage and it works with 4.20.x.
That makes me wonder if this app will work with 4.20.x. But, it could easily be another issue that breaks it. I can't do a test at the moment, but, might have a play over the weekend.