As many have surmised, the recent change in the WaniKani v2 API is causing AlliCrab to fail. I was intentionally strict in the parsing of data from the API, which is causing the unfortunately vague error message when a property which was indicated to always be present is missing. (In any case, the app uses the resurrected property which was removed.)
This will require a code change to fix, and unfortunately as I have previously mentioned on this thread I am not able to release new versions of the app. So rather than leaving a broken version on the store, I have decided to remove it.
Thank you all for your support these past four years.