The last version of KanjiBox (2.1.3 currently in the store) finally cut support for iOS 3 and even jumped to requiring iOS 4.3. This was based on my (mistaken) assumption that all devices (except for 1G devices: stuck on 3.x) could at least run 4.3.
As it turns out, 2G devices (iPhone and iPod Touch 2nd generation) can only go as high as iOS 4.2.1.
Since it was reasonably little work to add back support for iOS 4.2, I just made the necessary changes, and a new update (KanjiBox 2.1.4) should soon be in the store, that supports all devices 2G and up.
Now if you are curious as to the reasons why KanjiBox is routinely forced to drop support for older devices (and why I love you but Apple loves your wallet more), feel free to read the rantish explanation below: