LDAP users/groups not visible/usable in nextcloud 12

Nextcloud version (eg, 10.0.2): 12.0.0
Operating system and version (eg, Ubuntu 16.04): Debian Jessie (kernel 3.16.43-2)
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.10 (Debian)
PHP version (eg, 5.6): PHP 7.0.20-1~dotdeb+8.1
Is this the first time you’ve seen this error?:

Can you reliably replicate it? (If so, please outline steps):
unknown, but by reading other people’s issues with LDAP, it would most likely be reproducable

The issue you are facing:
I’ve added the LDAP connection and users are able to login to nextcloud with their LDAP account.
Those people do see their group membership when going to their ‘personal’ page. These groups are all LDAP groups
When such a person clicks on the Addressbook (at the top of the page, other contacts are listed their, but they are not clickable, don’t know if they should be btw).
When such a person wants to share, for example a folder, it can’t find any contact and when trying to share to a LDAP group it gets the message that the user isn’t a member of that group (which he is according to his ‘personal’ page in nextcloud).

There are also no users or groups visible when an (local) admin logs in into nextcloud and goes to the user management page.

The output of your Nextcloud log in Admin > Logging:
No new messages appear when a user logs in
No new messages appear when a user tries to share a directory

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

<?php $CONFIG = array ( 'passwordsalt' => 'REMOVED', 'secret' => '/REMOVED', 'trusted_domains' => array ( 0 => 'REMOVED', ), 'datadirectory' => '/REMOVED/', 'overwrite.cli.url' => 'http://localhost', 'dbtype' => 'mysql', 'version' => '', 'dbname' => 'REMOVED', 'dbhost' => 'REMOVED', 'dbport' => '', 'dbtableprefix' => 'oc_', 'dbuser' => 'REMOVED', 'dbpassword' => 'REMOVED', 'logtimezone' => 'UTC', 'installed' => true, 'instanceid' => 'ocrqc7pv3rdn', 'mail_smtpmode' => 'smtp', 'mail_from_address' => 'REMOVED', 'mail_domain' => 'REMOVED', 'mail_smtphost' => '', 'mail_smtpport' => '25', 'memcache.local' => '\\OC\\Memcache\\Redis', 'memcache.locking' => '\\OC\\Memcache\\Redis', 'redis' => array ( 'host' => 'REMOVED', 'port' => REMOVED, ), 'ldapIgnoreNamingRules' => false, 'ldapProviderFactory' => '\\OCA\\User_LDAP\\LDAPProviderFactory', 'updater.release.channel' => 'production', 'updater.secret' => 'REMOVED', 'maintenance' => false, 'theme' => '', 'loglevel' => 2, ); The output of your Apache/nginx/system log in `/var/log/____`: