Hi @kfincei !
First I don’t see that the issue you discribe in your first post is identical to the issue discribed in the Github post mentioned by @irgendwie in the second post ( https://github.com/nextcloud/contacts/issues/399 ).
I can reproduce the error happening when I create a second address book and try to sync to the Mac (I think it is well known that Mac treatment of multiple address books as well as groups is not conforming to the CalDAV standard, while iOS does).
On the other hand, I can’t reproduce the issue that you describe above: I have created a test contact on my iPhone, which synced without any problem to my Mac (10.13.1).
So my first question would be, do you have multiple address books on your NextCloud account?