App Store problems
As you may or may not have heard, the App Store is currently experiencing problems with app updates. Unfortunately Convertible appears to be one of the apps affected. The problem is causing some apps not to load after being updated with the only fix being to delete the app and re-install it.
I have temporarily changed the price of Convertible to free while these problems continue, to help anyone who may have trouble re-installing the app.
I will update this post once Apple resolve the issue.
Update: It appears the problem has been resolved.