[Samba] Cannot join server to Samba4 NT4 domain

[MI]

I have a Samba 4 NT4 PDC (Version 4.1.17-Debian) with openLDAP.

I would like to add another server, and have it authenticate users against openLDAP. 
I thought I had to add the new server to the domain with "net rpc join", but that 
seems to think I want to join an AD domain, and fails:

     # net rpc join -U root%mypassword
     No realm has been specified! Do you really want to join an Active Directory server?
     Failed to join domain: failed to lookup DC info for domain 'MYDOMAIN' over rpc: 
This error indicates that the requested
     operation cannot be completed due to a catastrophic media failure or an on-disk 
data structure corruption.

Before that, I tried to configure it just as a standalone server with LDAP, but that 
didn't work either (it didn't find the user accounts)

Would someone know how to add a plain file server to a Samba 4 domain, and have the 
file server authenticate the LDAP users?

Below is my current config which gives the "net rpc join" error above:

# testparm -s
Load smb config files from /etc/samba/smb.conf
Processing section "[backups]"
Processing section "[diskimages]"
Loaded services file OK.
Server role: ROLE_DOMAIN_MEMBER

# Global parameters
[global]
     workgroup = MYDOMAIN
     server role = standalone server
     security = DOMAIN
     map to guest = Bad User
     obey pam restrictions = Yes
     passdb backend = ldapsam:"ldap://localhost ldap://ldap.mydomain.lan 
ldap://ldap2.mydomaini.lan"
     pam password change = Yes
     passwd program = /usr/bin/passwd %u
     passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n 
*password\supdated\ssuccessfully* .
     unix password sync = Yes
     syslog = 0
     log file = /var/log/samba/log.%m
     max log size = 4000
     dns proxy = No
     wins server = 192.168.44.10
     ldap admin dn = "cn=admin,dc=mydomain,dc=lan"
     ldap group suffix = ou=Groups
     ldap machine suffix = ou=Computers
     ldap suffix = dc=mydomain,dc=lan
     ldap ssl = no
     ldap user suffix = ou=People
     usershare allow guests = Yes
     panic action = /usr/share/samba/panic-action %d
     idmap config * : backend = tdb


[backups]
...


In case it matters, this is the PDC config:

# testparm -s
Load smb config files from /etc/samba/smb.conf
ldap_url_parse_ext(ldap://localhost/)
ldap_init: trying /etc/ldap/ldap.conf
ldap_init: using /etc/ldap/ldap.conf
ldap_url_parse_ext(ldapi://)
ldap_url_parse_ext(ldap://)
ldap_init: HOME env is /root
ldap_init: trying /root/ldaprc
ldap_init: trying /root/.ldaprc
ldap_init: trying ldaprc
ldap_init: LDAPCONF env is NULL
ldap_init: LDAPRC env is NULL
Processing section "[netlogon]"
...
Loaded services file OK.
Server role: ROLE_DOMAIN_PDC
[global]
     workgroup = MYDOMAIN
     netbios name = JANUS
     server string = %h server
     interfaces = 127.0.0.0/8, 192.168.44.10/24, 10.44.0.0/24
     server role = classic primary domain controller
     map to guest = Bad User
     passdb backend = ldapsam
     syslog = 0
     log file = /var/log/samba/log.%m
     server max protocol = NT1
     time server = Yes
     unix extensions = No
     load printers = No
     printcap name = /dev/null
     disable spoolss = Yes
     show add printer wizard = No
     add machine script = /usr/sbin/smbldap-useradd -w "%u"
     logon script = logon-%a.bat
     logon path = \\%N\%U\profile-%a
     logon drive = H:
     domain logons = Yes
     os level = 64
     preferred master = Yes
     domain master = Yes
     wins support = Yes
     ldap admin dn = "cn=admin,dc=mydomain,dc=lan"
     ldap group suffix = ou=Groups
     ldap machine suffix = ou=Computers
     ldap passwd sync = yes
     ldap suffix = dc=frenetic,dc=lan
     ldap ssl = no
     ldap user suffix = ou=People
     ldap debug level = 1
     panic action = /usr/share/samba/panic-action %d
     ldapsam:trusted = yes
     idmap config * : backend = tdb
     acl allow execute always = Yes
     create mask = 0775
     directory mask = 02775
     force unknown acl user = Yes
     print notify backchannel = No
     printing = bsd
     print command = lpr -r -P'%p' %s
     lpq command = lpq -P'%p'
     lprm command = lprm -P'%p' %j
     veto oplock files = /*.doc*/*.DOC*/*.xls*/*.XLS*/*.mdb/*.MDB/~$*/
     csc policy = disable

[netlogon]
...