[Samba] Samba didn't find groups in LDAP

[Theo]

Good evening,

I'm running Samba 4.1.6 and OpenLDAP 2.4.31 on an Ubuntu 14.04 box.
Since yesterday, Samba isn't able anymore to find my groups in the LDAP
directory tree. I have no idea about this behavior; here are some lines
copied from /var/log/samba/log.smbd:

*** BEGIN ***

ldap_connect_system: successful connection to the LDAP server
Got user=[om] domain=[OEGNET] workstation=[HERMES] len1=24 len2=142
init_sam_from_ldap: Entry found for user: om
ERROR: Got 0 entries for gid 1003, expected one
primary group of [om] not found
Failed to get groups from sam account.
check_sam_security: make_server_info_sam() failed with
'NT_STATUS_INTERNAL_DB_CORRUPTION'

*** END ***

I omitted some lines with lower importance.

My user account (om) has been found, my gidNumber (1003) is correct, but
you see the error above.

Can someone help, please? If you need further information, please let me
know.

Kind regards, Theo