Unable to authenticate using LDAPS

Hi,

I am somehow unable to login to my Giutlab server using LDAPS. This is what I see on the webinterface: (the username and password are definitely the right ones):
image

This is my LDAP-Configuration in the gitlab.rb file:

gitlab_rails['ldap_enabled'] = true

gitlab_rails['ldap_servers'] = {
'main' => {
  'label' => 'LDAP',
  'host' =>  'novo-ads-1.domain.local',
  'port' => 636,
  'uid' => 'sAMAccountName',
  'encryption' => 'simple_tls',
  'verify_certificates' => false,
  'bind_dn' => 'ldapro@domain.local',
  'password' => 'superawesomepass',
  'active_directory' => true,
  'base' => 'OU=ABT,DC=domain,DC=local',
  'group_base' => '',
  'admin_group' => ''
  }
}

This is my output, when I try to test it on the console:

root@git:~# gitlab-rake gitlab:ldap:check
Checking LDAP ...

LDAP: ... Server: ldapmain
not verifying SSL hostname of LDAPS server 'novo-ads-1.domain.local:636'
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
        DN: cn=weiske mandy,ou=ps,ou=gf,ou=abt,dc=domain,dc=local  sAMAccountName: weiske
        DN: cn=lohnpruefer,ou=ps,ou=gf,ou=abt,dc=domain,dc=local   sAMAccountName: Lohnpruefer
		...
        DN: cn=novozugang-ssl,ou=it,ou=abt,dc=domain,dc=local      sAMAccountName: novozugang-ssl
        DN: cn=czemmel admin,ou=it,ou=abt,dc=domain,dc=local       sAMAccountName: czemmel-admin

Checking LDAP ... Finished

Any idea what might cause this or how to debug this further?