To domain join the OpenVpn server do the following:

Configure the Server to use LDAP:

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')