Took me a while but I solved not being able to sync carddav on my setup. Apologies if this has been covered before, I havenāt been it.
Preface: Running NC10 on docker/pi with nginx on a non-standard port, though this could help you solve like issues. Carddav never worked for me so every upgrade I spend some time to try again.
My situation is that whether adding an account for the Contacts > Prefs > Accounts or Sys Prefs > Internet Accounts and whether I chose Auto to Advanced, I got a ā Cannot verify username and password ā or some such error.
Anyhoo, I happened along to the error log for the accountsd process and it kept complaining about not being able to access the server using almost none of the details I was actually typing in. 
Your details may differ but basically I took apart the address it was trying to hit and made redirects in docker. For me the error was,
... accountsd[482]: Could not verify credentials for account ... NSErrorFailingURLKey=https://<username>@<serverip>:8843/
Not sure why it would not honor my specific port and would only check rootā¦
In the end all it took was a new block added in to nginx.conf,
server {
listen 8443;
server_name <servername>;
location = /.well-known/carddav { return 301 $scheme://$host:<realport>/remote.php/dav; }
location = /.well-known/caldav { return 301 $scheme://$host:<realport>/remote.php/dav; }
}
If you try this, please replace the and with the details of where your next cloud instance is running. The ālisten 8443;ā in the nginx block is from the Mac console error log so if you look there and want to try this, please use that port number.
Regards-
James