When using an iOS device with an operation system version of iOS 10.3.x, it may occur that the backup functionality provided by apple is not useable.
With the update to iOS 10.3. Apple implemented a new file system on its devices called APFS.
This system needs specifing tweaking towards the useage of umlauts. If the apps are not tweaked it may occur that files with umlauts are not handled properly and result in error messages or app crashes.
Following this the backup system might be interrupted and produce an error message.
This bug was resolved with iOS 11. The backup functionality can properly be used after an update to iOS 11.
Update your device to iOS 11 or higher.
We are not able to provide an update for the Onleihe-App using the iOS Verison 10.3.x.