Openldap 2.4.11 – bdb_dn2id: get failed: DB_NOTFOUND: No matching key/data pair found (-30989)

I am struggling quite a while now, googled a lot and I have not yet been able to figure this – hopefully – simple problem out. I am a beginner in ldap so I might be missing something very obvious …
I have a pretty simple slapd.conf:

include     /private/etc/openldap/schema/core.schema
pidfile     /private/var/db/openldap/run/slapd.pid
argsfile    /private/var/db/openldap/run/slapd.args
database    bdb
suffix      "dc=ronr,dc=nl"
rootdn      "cn=manager,dc=ronr,dc=nl"
rootpw      zz
directory   /private/var/db/openldap/openldap-data
index   objectClass eq

and a ldap.conf:

URI ldap://localhost

and started slapd.
When trying to connect to the rootdn using

ldapadd -x -D "cn=manager,dc=ronr,dc=nl" -w zz -f add_base.ldif -h localhost -d255

I get:
ldap_bind: Invalid credentials (49)

In the debug level 250 trace of slapd I get:

slapd startup: initiated.
backend_startup_one: starting "cn=config"
config_back_db_open
config_build_entry: "cn=config"
config_build_entry: "cn=schema"
config_build_entry: "cn={0}core"
config_build_entry: "olcDatabase={-1}frontend"
config_build_entry: "olcDatabase={0}config"
config_build_entry: "olcDatabase={1}bdb"
backend_startup_one: starting "dc=ronr,dc=nl"
bdb_db_open: database "dc=ronr,dc=nl": dbenv_open(/private/var/db/openldap/openldap-data).
bdb_monitor_db_open: monitoring disabled; configure monitor database to enable
slapd starting
slap_listener_activate(7): 
>>> slap_listener(ldap:///)
connection_get(13): got connid=0
connection_read(13): checking for input on id=0
ber_get_next
ber_get_next: tag 0x30 len 38 contents:
ber_get_next
conn=0 op=0 do_bind
ber_scanf fmt ({imt) ber:
ber_scanf fmt (m}) ber:
>>> dnPrettyNormal: <cn=manager,dc=ronr,dc=nl>
<<< dnPrettyNormal: <cn=manager,dc=ronr,dc=nl>, <cn=manager,dc=ronr,dc=nl>
do_bind: version=3 dn="cn=manager,dc=ronr,dc=nl" method=128
bdb_dn2entry("cn=manager,dc=ronr,dc=nl")
=> bdb_dn2id("dc=ronr,dc=nl")
**<= bdb_dn2id: get failed: DB_NOTFOUND: No matching key/data pair found (-30989)**
send_ldap_result: conn=0 op=0 p=3
send_ldap_response: msgid=1 tag=97 err=49
ber_flush2: 14 bytes to sd 13
connection_get(13): got connid=0
connection_read(13): checking for input on id=0
ber_get_next
ber_get_next on fd 13 failed errno=0 (Undefined error: 0)
connection_closing: readying conn=0 sd=13 for close
connection_close: conn=0 sd=13

I suspect that the line

<= bdb_dn2id: get failed: DB_NOTFOUND: No matching key/data pair found (-30989)

is trying to tell me the real problem. Point is I have no idea what is missing here.
How can I fix this?
Should I do something extra to administer the password, other than just putting it in the slapd.conf as I did, if so, what should I do?
FWIW: I am running this on MacOSX (10.6.6)

any helpful tip/suggestion is very much appreciated.
Ronald.