systemd, override, slapd daemon fails

I've tried to override the standard slapd (OpenLDAP) daemon start parameters with systemd, but as soon as I override the ExecStart, the daemon fails to start. My questions is why it fails and how I can change the start parameters of the daemon?

I've overrided systemd slapd.service file with:

root@debian:~ $ systemctl edit slapd
[Service]
ExecStart=
ExecStart=/usr/sbin/slapd -h "ldap:/// ldaps:/// ldapi:///" -g openldap -u openldap -F /etc/ldap/slapd.d

Here just a few things to debug:

root@debian:~ $ systemctl cat slapd
# /run/systemd/generator.late/slapd.service
# Automatically generated by systemd-sysv-generator

[Unit]
Documentation=man:systemd-sysv-generator(8)
SourcePath=/etc/init.d/slapd
Description=LSB: OpenLDAP standalone server (Lightweight Directory Access Protoc
Before=multi-user.target
Before=multi-user.target
Before=multi-user.target
Before=graphical.target
After=remote-fs.target
After=network-online.target
Wants=network-online.target

[Service]
Type=forking
Restart=no
TimeoutSec=5min
IgnoreSIGPIPE=no
KillMode=process
GuessMainPID=no
RemainAfterExit=yes
SuccessExitStatus=5 6
ExecStart=/etc/init.d/slapd start
ExecStop=/etc/init.d/slapd stop

Running the daemon without overriding show:

root@debian:~ $ systemctl status slapd
● slapd.service - LSB: OpenLDAP standalone server (Lightweight Directory Access Protocol)
   Loaded: loaded (/etc/init.d/slapd; generated; vendor preset: enabled)
   Active: active (running) since Mon 2018-07-30 11:33:40 CEST; 1h 20min ago
     Docs: man:systemd-sysv-generator(8)
  Process: 429 ExecStart=/etc/init.d/slapd start (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/slapd.service
           └─509 /usr/sbin/slapd -h ldap:/// ldapi:/// -g openldap -u openldap -F /etc/ldap/slapd.d

Running the daemon with overrriding, after "systemctl daemon-reload" and "systemctl restart slapd" throws the following errrors:

root@debian:~ $ sudo systemctl status slapd
● slapd.service - LSB: OpenLDAP standalone server (Lightweight Directory Access Protocol)
   Loaded: loaded (/etc/init.d/slapd; generated; vendor preset: enabled)
  Drop-In: /etc/systemd/system/slapd.service.d
           └─override.conf
   Active: failed (Result: exit-code) since Mon 2018-07-30 13:50:13 CEST; 37s ago
     Docs: man:systemd-sysv-generator(8)

Jul 30 13:50:11 udamc systemd[1]: Starting LSB: OpenLDAP standalone server (Lightweight Directory Access Protocol)...
Jul 30 13:50:13 udamc slapd[438]: @(#) $OpenLDAP: slapd  (Aug 10 2017 19:12:46) $
                                          Debian OpenLDAP Maintainers <pkg-openldap-devel@lists.alioth.debian.org>
Jul 30 13:50:13 udamc slapd[438]: daemon: bind(9) failed errno=2 (No such file or directory)
Jul 30 13:50:13 udamc slapd[438]: slapd stopped.
Jul 30 13:50:13 udamc slapd[438]: connections_destroy: nothing to destroy.
Jul 30 13:50:13 udamc systemd[1]: slapd.service: Control process exited, code=exited status=1
Jul 30 13:50:13 udamc systemd[1]: Failed to start LSB: OpenLDAP standalone server (Lightweight Directory Access Protocol).
Jul 30 13:50:13 udamc systemd[1]: slapd.service: Unit entered failed state.
Jul 30 13:50:13 udamc systemd[1]: slapd.service: Failed with result 'exit-code'.