Do your clients use an app, which was written by you? Let me suggest you to test your app on Android 14, there are also emulators, and adjust your app accordingly…
Which ftp extension are you using? Ftp generally is working fine, but most probably your clients like to access files, the app does not own? Read Some basics on Android storage system
MANAGE_EXTERNAL_STORAGE permission is only a solution if you do not use Google Play to distribute your app…
Taifun