my slapd.conf now looks like (now in more detail, just cleaned up):
--- slapd.conf
...
modulepath /usr/lib/openldap/modules
moduleload smbk5pwd.so
sizelimit unlimited
acl ...
TLSstuff ...
#### chain overlay definition
overlay chain
chain-rebind-as-user FALSE
chain-uri "ldaps://ldapprov"
chain-rebind-as-user TRUE
chain-idassert-bind bindmethod="simple"
binddn="cn=manager,o=test"
credentials="secret"
mode="self"
database bdb
suffix "o=test"
directory /var/lib/ldap/
rootdn "cn=manager,o=test"
rootpw "secret"
index objectClass,uidNumber,gidNumber eq
index member,mail eq,pres
index cn,displayname,uid,sn,givenname sub,eq,pres
index sambaSID,sambaPrimaryGroupSID,sambaDomainName eq
index entryCSN,entryUUID eq
index dhcpHWAddress eq,pres
index relativeDomainName eq,pres
index ipHostNumber eq,pres
index zoneName eq,pres
index radiusGroupName eq,pres
syncrepl rid=13
provider=ldaps://ldapprov
type=refreshAndPersist
retry=1,5,5,6,30,+
interval=00:00:00:30
searchbase="o=test"
filter="(objectclass=*)"
scope=sub
attrs="*"
schemachecking=off
binddn="cn=manager,o=test"
bindmethod=simple
credentials="secret"
sizelimit=unlimited
updateref ldaps://ldapprov
overlay syncprov
overlay smbk5pwd
smbk5pwd-enable samba
--- end of slapd.conf