Yesterday i fixed a problem with the contacts app on my mac. It said, “some Contacts couldn't be synced” and didn't sync some of my contacts, most of them where still synced. To invesitgate, I looked in the Console.app and filtered for Contacts. It said:

2017-06-20 23:11:35.008086+0200 0x17f462   Fault       0x80000000000545f6   26290  Contacts: (CardDAVPlugin) [com.apple.contacts.carddav.plugin] -_handleAddsOrModifies: could not parse vCard in <private>

and

2017-06-20 23:16:07.355156+0200 0x182af7   Fault       0x8000000000054af8   28288  Contacts: (CardDAVPlugin) [com.apple.contacts.carddav.plugin] CardInfoFromVCardData(): Ignored vCard <private>:<private> because ABPerson create failed. VCard: "<private>"

so i knew there was more info for my problem, but it was hidden behind the <private>. How to change this behavior i decribed in an earlier post. After that you can simply backup the data of this card, delete it from the source device and recreate it, this should fix your problem.