Apple has implemented new permission restrictions in iOS 8.3: access to application data is restricted to the "Documents" folder of apps that have the "File Sharing" feature explicitly enabled by the developers. Note that application data may still be browsed and extracted via iMazing's backup browser, File System section - just remember to backup your device first for access to the most up to date data.
If you need to access a particular app's data directly and are unable to do so, you should contact the developer of the app and request that they enable "File Sharing" and store the data you wish to access in the "Documents" folder of their application. This is what our file manager FileApp does, for example.
Note to developers: all other folders such as "Library" or "tmp" are still accessible for in development apps.
For Developers: Enabling File Sharing
As explained on the following article on StackOverflow:
"You just have to set UIFileSharingEnabled (Application Supports iTunes file sharing) key in the info plist of your app. Here's a link for the documentation. Scroll down to the file sharing support part.
You must also define CFBundleDisplayName (Bundle Display Name), if it isn't already there. More details here."