sync server has failed to sync the iphone

I had the same issue with an iPhone 6 and Outlook. In iTunes, I clicked on replace the contacts and calendar with the data in outlook and then hit Sync. It worked and now I am syncing normally again.

Plug your iPhone into your PC, activating the iTunes sync feature.

Click on the iPhone symbol on the left side of the screen in the title bar.

Under Settings on the left side, click on "Info".

In the middle of the screen under "Advanced" you should see a box to click next to "Contacts" and "Calendars" which will replace the information on your iPhone with information from Outlook stored on your computer.

I'm not sure this will work for you, but it did for me. After replacing the information on the iPhone this one time, it now syncs properly when plugged in to the PC. Good luck.

Error Message: "iTunes could not sync contacts to the iPhone because the sync server failed to sync the iPhone". This is a recent error I can't seem to fix or find a reasonable solution to. The sync feature has worked without error since buying the iPhone 6 a few years ago. Any thoughts?

What you can sync with iTunes (**Only if they are NOT in sync with iCloud)

• Albums, songs, playlists, movies, TV shows, podcasts, books, and audiobooks.
• Photos and videos**
• Contacts and calendars**
• Device backups that you made using iTunes.

iTunes features might vary by country or region.

Sync or remove content using iTunes

1. Open iTunes and connect your device to your computer with a USB cable.
2. Click the device icon in the upper-left corner of the iTunes window. Learn what to do if you don't see the icon. 
3. From the list under Settings on the left side of the iTunes window, click the type of content that you want to sync or remove.1 To turn syncing on for a content type, select the checkbox next to Sync.
4. Select the checkbox next to each item that you want to sync.
5. Click the Apply button in the lower-right corner of the screen.
6. If syncing doesn't start automatically, click the Sync button.

1. You can only sync your iOS device with one iTunes library at a time. If you see a message that your iOS device is synced with another iTunes library, your iOS device was previously connected to another computer. If you click "Erase and Sync" in that message, all content of the selected type on your iOS device will be erased and replaced with content from this computer.

Sync your content using Wi-Fi

After you set up syncing with iTunes using USB, you can set up iTunes to sync to your device with Wi-Fi instead of USB. Here's how:

1. Connect your iOS device to your computer with a USB cable, then open iTunes and select your device. 
2. Click Summary on the left side of the iTunes window. 
3. Select "Sync with this [device] over Wi-Fi." 
4. Click Apply.

When the computer and the iOS device are on the same Wi-Fi network, the iOS device appears in iTunes. The iOS device syncs automatically whenever it's plugged in to power and iTunes is open on the computer.

Sync your iPhone, iPad, or iPod using iTunes on your computer ...

https://support.apple.com/en-in/HT201253