I fixed this issue as follows:
Complete remove of OpenLDAP using
sudo service slapd stop
sudo apt-get -y remove --purge slapd
rm -rf /var/lib/ldap
sudo rm -rf /etc/ldap/
sudo apt-get -y remove --purge ldap-utils
Install + restore of backup using script similar to
#!/bin/bash
#
set -x #echo on
export LC_ALL=en_US.UTF-8
export DEBIAN_FRONTEND=noninteractive
echo -e " \
slapd slapd/internal/generated_adminpw password ******
slapd slapd/password2 password ******
slapd slapd/internal/adminpw password ******
slapd slapd/password1 password ******
slapd slapd/allow_ldap_v2 boolean false
slapd slapd/move_old_database boolean true
slapd slapd/dump_database_destdir string /var/backups/slapd-VERSION
slapd slapd/domain string mydomain.com
slapd slapd/dump_database select when needed
slapd slapd/invalid_config boolean true
slapd slapd/no_configuration boolean false
slapd shared/organization string MyDomain.com
slapd slapd/backend select HDB
slapd slapd/purge_database boolean true
slapd slapd/upgrade_slapcat_failure error
slapd slapd/password_mismatch note" | sudo debconf-set-selections
sudo apt-get install -y slapd ldap-utils
sudo usermod -a -G openldap user # add user user to openldap group
sudo service slapd status
sudo service slapd start
sudo slapcat
# change root-pw
cat <<EOT >> /home/ostraaten/1-olc-root-pw.ldif
dn: olcDatabase={1}hdb,cn=config
replace: olcRootPW
olcRootPW: {SSHA}******
EOT
sudo ldapmodify -Y EXTERNAL -H ldapi:/// -f 1-olc-root-pw.ldif
ldapwhoami -vvv -h localhost -p 389 -D cn=admin,dc=mydomain,dc=com -x -w ******
# add memberof attribuut
cat <<EOT >> 2-overlay.ldif
dn: cn=module,cn=config
objectClass: olcModuleList
cn: module
olcModulePath: /usr/lib/ldap
olcModuleLoad: memberof
dn: olcOverlay=memberof,olcDatabase={1}hdb,cn=config
objectClass: olcMemberOf
objectClass: olcOverlayConfig
objectClass: olcConfig
objectClass: top
olcOverlay: memberof
olcMemberOfDangling: ignore
olcMemberOfRefInt: TRUE
olcMemberOfGroupOC: groupOfNames
olcMemberOfMemberAD: member
olcMemberOfMemberOfAD: memberOf
EOT
sudo ldapadd -Y EXTERNAL -H ldapi:/// -f 2-overlay.ldif
# create schema using backup ldif
cat <<EOT >> 3-ok.ldif
dn: ou=people,dc=mydomain,dc=com
objectClass: organizationalUnit
objectClass: top
ou: people
dn: ou=groups,dc=mydomain,dc=com
objectClass: organizationalUnit
objectClass: top
ou: groups
# etc etc
EOT
ldapadd -h localhost -p 389 -D cn=admin,dc=mydomain,dc=com -w ****** -f 3-ok.ldif