Stop slapd:

systemctl stop slapd

Check LDAP directory and delete files in directory below, except DB_CONFIG:

# find openldap directory
grep ^directory /etc/openldap/slapd.conf
directory   /var/lib/ldap/<yourdomain>

# delete files from directory
cd /var/lib/ldap/<yourdomain>
shopt -s extglob
rm -f !(DB_CONFIG)

Start and immediately stop slapd to recreate nessecary files:

systemctl start slapd
systemctl stop slapd

Make sure OpenLDAP is NOT running, then restore from LDIF:

slapadd -f /etc/openldap/slapd.conf -l /path/to/backup/backup.ldif

Start slapd:

systemctl start slapd

Set correct permissions and restart slapd:

chown ldap. /var/lib/ldap/<yourdomain>/*.bdb
systemctl restart slapd

Change passwords for vmail and vmailadmin (see iRedMail.tips file):

#cn=vmail,dc=xxx,dc=xxx
#cn=vmailadmin,dc=xxx,dc=xxx

ldappasswd -x -h localhost -D "cn=Manager,dc=<yourdomain>,dc=com" -w <rootpass> -s <userpass> "cn=vmail,dc=<yourdomain>,dc=com"
ldappasswd -x -h localhost -D "cn=Manager,dc=<yourdomain>,dc=com" -w <rootpass> -s <userpass> "cn=vmailadmin,dc=<yourdomain>,dc=com"

Verify example:

ldapsearch -D "cn=Manager,dc=<yourdomain>,dc=com" -w password -h localhost -b "dc=<yourdomain>,dc=com"