This problem can be fixed by uninstalling and reinstalling the BoxCryptor app. I'm very sorry, but the latest update introduced a small bug with a huge impact when you update the app. It only occurs when the app is being updated from a previous version and does not occur on new installations.
We have submitted v1.2.2 to Apple already which should fix this problem and we're crossing our fingers that Apple approves it as soon as possible.
If you're interested, here's a more technical background on the problem:
In v1.2 the app did store the location of the cache folder in the shared preferences. When the app is updated, the location of the cache folder changes, but instead of querying the new cache folder, the app reads the old value from the shared preferences and tries to write to the old cache location which is (correctly) denied by iOS.
Result: The app cannot be used anymore, because writing to the cache folder is required. When trying to use the app, you get the error message with code 4.
Resolution: The app now always queries the current location of the cache and does not store the result anymore.