The solution is to reset the setting on your iPhone or your iPad. I mean reset in the very meaning of the word.
I don't use Windows, so this is tested only with Mac OS (10.8.5).
1. Connect your mobile device to the usb-port of your Mac.
2. In iTunes switch to your device.
3. On the summary-page (first page) uncheck the option for wireless syncing ("Sync with this iPad/iPhone over Wi-Fi")
4. Press the button "Apply"
5. Press the button "Sync"
6. Check the option for wireless syncing again.
7. Press the button "Apply"
8. Press the button "Sync"
That's it. This worked for me.
As of now your iTunes-Wi-Fi-Sync is active again on your mobile device.