Originally, the only development language that Apple provided for iOS was a proprietary one called Objective-C. All apps, including the Sales app for iOS, were written in this language. A few years ago, Apple introduced a new language for iOS called Swift. Converting the entire app to Swift was not an option, so it was largely ignored.
The iOS app includes support for Dropbox, using the SDK for the then current API v1. Recently, Dropbox brought out API v2, and said that support for API v1 would cease on 6th June 2017. However, the SDK for API v1 only supports Swift, and not Objective-C. It suggests to use of various bridging tools, but this is not really a viable option.
Because of this, I have chosen to support another cloud platform instead, probably Google Drive. This will be added to the Android app in the first instance, and then carried across to iOS one. It is ironic that Google offers better support for iOS than Dropbox does, but that is the reality of the situation.
The name of the server used by the hosting service has changed, and so has its URL. The new name is used by Android version 5.4.0 and iOS version 6.2.0. The new Android release is available now, while the new iOS release is waiting for review.