To domain join the OpenVpn server do the following:

Configure the Server to use LDAP. Requiring the Following Active Directory Structure.

• basic.local
  • basic
    • Users → Admin
    • Groups → OpenVpnUsers

Login to the CLient using, without having the domain as prefix: ACHTUNG: having special characters like “.” makes OpenVPN escape them, so that they are not accepted. Check sAMAccountName=first\\2euser below

2018-02-12 16:17:42+0000 [-] LDAP invalid credentials on ldap://21.1.2.39/: {'info': '80090308: LdapErr: DSID-0C09042F, comment: AcceptSecurityContext error, data 52e, v2580', 'desc': 'Invalid credentials'} (facility='user_bind on u'CN=first.user,OU=Users,OU=basic,DC=basic,DC=local' via search (u'OU=Users, OU=basic,  DC=basic, DC=local', 2, '(sAMAccountName=first\\2euser)')') (user='first.user')

To Debug check the logs: cat /var/log/openvpnas.log

Means the LDAP bind user credentials ae wrong. Here basic.local\Admin:

2018-02-12 16:15:53+0000 [-] LDAP invalid credentials on ldap://21.1.2.39/: {'info': '80090308: LdapErr: DSID-0C09042F, comment: AcceptSecurityContext error, data 52e, v2580', 'desc': 'Invalid credentials'} (facility='admin_bind to [basic.local\Admin]') (user='first.user')

Means the user credentials are wrong. Here s000001

LDAP invalid credentials on ldap://21.1.2.39/: {'info': '80090308: LdapErr: DSID-0C09042F, comment: AcceptSecurityContext error, data 52e, v2580', 'desc': 'Invalid credentials'} (facility='user_bind on u'CN=s000001,OU=Users,OU=basic,DC=basic,DC=local' via search (u'OU=Users, OU=basic,  DC=basic, DC=local', 2, '(sAMAccountName=s000001)')') (user='s000001')

$ cat /usr/local/openvpn_as/etc/config.json
{
  "Default": {
    "admin_ui.https.ip_address": "all",
    "admin_ui.https.port": "943",
    "auth.ldap.0.name": "My LDAP servers",
    "auth.ldap.0.ssl_verify": "never",
    "auth.ldap.0.timeout": "4",
    "auth.ldap.0.use_ssl": "never",
    "auth.module.type": "local",
    "auth.pam.0.service": "openvpnas",
    "auth.radius.0.acct_enable": "false",
    "auth.radius.0.name": "My Radius servers",
    "cs.cws_proto_v2": "true",
    "cs.https.ip_address": "all",
    "cs.https.port": "943",
    "cs.prof_sign_web": "true",
    "host.name": "34.244.71.201",
    "sa.initial_run_groups.0": "web_group",
    "sa.initial_run_groups.1": "openvpn_group",
    "vpn.client.basic": "false",
    "vpn.client.config_text": "cipher AES-128-CBC",
    "vpn.client.routing.inter_client": "false",
    "vpn.client.routing.reroute_dns": "false",
    "vpn.client.routing.reroute_gw": "false",
    "vpn.daemon.0.client.netmask_bits": "20",
    "vpn.daemon.0.client.network": "172.27.224.0",
    "vpn.daemon.0.listen.ip_address": "all",
    "vpn.daemon.0.listen.port": "443",
    "vpn.daemon.0.listen.protocol": "tcp",
    "vpn.daemon.0.server.ip_address": "all",
    "vpn.server.config_text": "cipher AES-128-CBC",
    "vpn.server.daemon.enable": "true",
    "vpn.server.daemon.tcp.n_daemons": 1,
    "vpn.server.daemon.tcp.port": "443",
    "vpn.server.daemon.udp.n_daemons": 1,
    "vpn.server.daemon.udp.port": "1194",
    "vpn.server.group_pool.0": "172.27.240.0/20",
    "vpn.server.nat.masquerade": "true",
    "vpn.server.port_share.enable": "true",
    "vpn.server.port_share.ip_address": "1.2.3.4",
    "vpn.server.port_share.port": "1234",
    "vpn.server.port_share.service": "admin+client",
    "vpn.server.routing.private_access": "nat",
    "vpn.tls_refresh.do_reauth": "true",
    "vpn.tls_refresh.interval": "360"
  },
  "_INTERNAL": {
    "run_api.active_profile": "Default",
    "webui.edit_profile": "Default"
  }
}