I use accesslog too, try with it in both masters, I'm now using bdb, change it to your backend definition

Migue

allow bind_v2
include         /usr/local/openldap/etc/openldap/schema/core.schema
include        /usr/local/openldap/etc/openldap/schema/corba.schema
include        /usr/local/openldap/etc/openldap/schema/cosine.schema
include        /usr/local/openldap/etc/openldap/schema/nis.schema
include        /usr/local/openldap/etc/openldap/schema/inetorgperson.schema

pidfile         /usr/local/openldap/var/run/slapd.pid
argsfile        /usr/local/openldap/var/run/slapd.args

modulepath      /usr/local/openldap/libexec/openldap
moduleload syncprov
password-hash {SSHA}

database bdb

suffix          cn=accesslog
rootdn          "cn=accesslog"
directory       "/usr/local/var/access"

index   default         eq
index   entryCSN eq
index   entryUUID eq
index   objectClass,reqEnd,reqResult,reqStart,reqDN,contextCSN

access to *
        by dn="cn=admin,dc=ar" read
        by * read

database        hdb
suffix          "dc=***,dc=com"
rootdn          "cn=root,dc=***,dc=com"
rootpw          {SSHA}yZkqhHmELfmUTsaQyfxgXBqq95gugTA4
directory       /usr/local/openldap/var/openldap-data

index uid pres,eq
index cn,sn pres,eq,approx,sub
index   objectClass     eq
index entryCSN,entryUUID eq

syncrepl rid=001
provider=ldap://192.168.1.12
type=refreshAndPersist
retry="5 5 300 +"
searchbase="dc=***,dc=com"
attrs=*
binddn="cn=root,dc=***,dc=com"
credentials=secret

overlay syncprov
syncprov-checkpoint 50 10

overlay accesslog
logdb cn=accesslog
logops writes
logsuccess TRUE
logpurge 2+00:00 1+00:00
logold (objectclass=auditDelete)
logoldattr reqDeleteOldRDN

database monitor
loglevel 256

mirrormode true