Restore important app. data from iOS 10 after an upgrade to iOS 11
That’s how I solved it:

Requirements


iPhone / iPad with iOS 11 running the password app which is not supported anymore
spare iPhone / iPad running iOS 10 with same password app freshly installed and no data inside


Procedure


download the latest free trial version of the iPhone Explorer
connect the iOS 11 device
go to Apps
select the password app
click MOUNT APP to display the appdata folder in a virtual drive with limited read / write access (Note: the trial version of iPhone explorer offers only 150MB of data to be written)
copy all files and folders to your local hard drive, make sure the folder structure is the same
connect the iOS 10 device
go to apps
select the empty password app
click MOUNT APP and copy over the iOS 11 appdata from your hard drive to the iOS 10 device
disconnect the iOS 10 device
open the password app and enter your master pin 
you should now see your passwords listed as usual


Hope that helps!