Contacts not syncing with OSX

Nextcloud version (eg, 12.0.2): 13.04
Operating system and version (eg, Ubuntu 17.04): 18.04
Apache or nginx version (eg, Apache 2.4.25): Apache
PHP version (eg, 7.1):7.2

The issue you are facing:

Created a CarDAV account in OSX, and successfully connected it.
I see the AccountName in the left column of the Contacts-App.

However, no contacts created in this account are being uploaded, nor are
contacts created in nextcloud are being downloaded.

The output of your Nextcloud log in Admin > Logging:

Fatal	webdav	Sabre\DAV\Exception\UnsupportedMediaType: This resource only supports valid vCard or jCard data. Parse error: End of document reached prematurely	2018-06-12T14:22:51+0200
Fatal	webdav	Sabre\Xml\ParseException: This should never happen (famous last words)	2018-06-12T13:17:37+0200
Fatal	webdav	Sabre\Xml\ParseException: This should never happen (famous last words)	2018-06-12T12:35:16+0200
Fatal	webdav	Sabre\Xml\ParseException: This should never happen (famous last words)	2018-06-12T12:25:32+0200
Fatal	webdav	Sabre\DAV\Exception\BadRequest: expected filesize 184990 got 180224	2018-06-12T05:09:13+0200
Fatal	webdav	OCA\DAV\Connector\Sabre\Exception\FileLocked: "admin/files/Xchange/joomla/old/templates/yoo_avenue/css/bootstrap.css.upload.part" is locked

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'instanceid' => 'xxxxxx',
  'passwordsalt' => 'xxxxxxx',
  'secret' => 'xxxxxxxxx',
  'trusted_domains' =>
  array (
    0 => 'xxxx.xxxxx.xxxx',
  ),
  'datadirectory' => '/var/www/clients/client0/web1/private/data',
  'overwrite.cli.url' => 'https://xxxx.xxxx.xxxx',
  'dbtype' => 'mysql',
  'version' => '13.0.4.0',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'nextclouduser',
  'dbpassword' => 'xxxxx',
  'installed' => true,
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'integrity.check.disabled' => true,
  'loglevel' => 4,
);