[Samba] Classicupgrade of 4.1.17 error: "list index out of range"

[MI]

I'm trying to upgrade to AD from a Samba 4 PDC with openLDAP (on Debian 8).

I'm stuck at this very unhelpful error:

    # samba-tool domain classicupgrade --dbdir=/usr/local/samba.PDC/dbdir/
    --use-xattrs=yes --realm=ad.mydomain.tld --dns-backend=SAMBA_INTERNAL
    --option="interfaces=lo eth2" --option="bind interfaces only=yes"
    /etc/samba/smb.conf.PDC
    Reading smb.conf
    Provisioning
    Exporting account policy
    Exporting groups
    Exporting users
       Skipping wellknown rid=501 (for username=nobody)
    Next rid = 3307
    ERROR(<type 'exceptions.IndexError'>): uncaught exception - list index out of range
       File "/usr/lib/python2.7/dist-packages/samba/netcmd/__init__.py", line 175, in
    _run
         return self.run(*args, **kwargs)
       File "/usr/lib/python2.7/dist-packages/samba/netcmd/domain.py", line 1321, in run
         useeadb=eadb, dns_backend=dns_backend, use_ntvfs=use_ntvfs)
       File "/usr/lib/python2.7/dist-packages/samba/upgrade.py", line 796, in
    upgrade_from_samba3
         urls = samba3.lp.get("passdb backend").split(":",1)[1].strip('"')


Any ideas on how to fix it or at least find some better clue?

    # samba --version
    Version 4.1.17-Debian

    # lsb_release -d
    Description:    Debian GNU/Linux 8.3 (jessie)



Thanks.