@Yetho
Vielleicht noch ein Ansatz:
On Windows
If you’re a Windows user and are having issues syncing your iOS devices with iTunes, you too can try resetting your Lockdown folder. These steps work for Windows Vista through to Windows 10:
1) Unplug all iOS devices from your computer.
2) Close iTunes so it’s not running in the background.
3) Press the Windows key on your keyboard or press Control + Esc at the same time.
4) Click inside of the search field and enter &ProgramData% and press the Enter key.
5) Open the Apple folder by double-clicking on it.
6) Right-click on the folder entitled Lockdown and Delete it.
7) Empty your Recycle Bin and restart your computer as well as your iOS devices.
Congratulations! You’ve just reset your iTunes Lockdown folder, and hopefully now you can start syncing and backing up your devices with your computer again.