Shortly after the update to iOS12 i recognised 2 problems. I was not able to connect my mac to my mobile hotspot and I was not able to connect the iPhone to my iTunes on the mac for synchronising. The reason is apple had to install a new component with iOS 12.x which will be requested via a popup in iTunes you can easily miss or which would not popup at every occasion.
To fix that problem you should
- download iTunes 12.8 from apples web site manually and install it.
- Afterwards, do a reboot.
- Then open up iTunes again. Connect your phone.
- The dialog will appear so follow the onscreen instructions.
- Ready to go again.