Problems that I’ve run into while trying to sync over wifi:
1. error -50
2. Required disc not found
While rebooting device / Mac seem to solve part of the problem, it’s just not stable.
Solution: reinstall iTunes.
A couple things to remember:
1. Check “sync over wifi-connection" in iTunes > Device > Summary – option
2. After checking the box, sync once (with USB plugged).
3. After sync has completed, unplug (NOT eject) the iPhone. You’ll find iPhone still on Device list even when it’s unplugged. Wifi sync now works.