Unable do slapadd a LDIF dump from slapd 2.2.6 to slapd 2.4.9

[Markus Wernig <listener@wernig.net>]

Hello all!

In the process of upgrading slapd from 2.2.6 to 2.4.9 (ubuntu 6.06 to 
8.04), I get the following error when trying to import the LDIF dump 
from the old server:

# slapadd -b "o=Company Name,c=CH" -l dump.from.2.2.6.ldif
slapadd: dn="o=Company Name,c=CH" (line=1): (65) no structural object 
class provided
(same result if I skip the -b "o=Company Name,c=CH")

slapd.conf has not been altered during the upgrade and contains:
[...]
include         /etc/ldap/schema/core.schema
include         /etc/ldap/schema/cosine.schema
include         /etc/ldap/schema/nis.schema
include         /etc/ldap/schema/inetorgperson.schema
[...]
database        bdb
suffix          "O=Company Name, C=CH"
checkpoint 512 30
directory       "/var/lib/ldap"
rootdn          "cn=root, O=Company Name, C=CH"
rootpw          XXX
index           objectClass     eq
[...]

The first entry in the LDIF dump is:
dn: o=Company Name,c=CH
objectClass: top
entryUUID: ...
creatorsName: cn=root,o=Company Name,c=CH
modifiersName: cn=root,o=Company Name,c=CH
createTimestamp: 20071214103903Z
modifyTimestamp: 20071214103903Z
entryCSN: 20071214103903Z#000001#00#000000

(This directory contains only x.509 CRLs and certificates.)

From the error I assumed that for some reason the "top" objectclass was 
not correctly defined. In core.schema I found the following new entry 
from RFC2256, which was commented out:

#objectclass ( 2.5.6.0 NAME 'top'
#       DESC 'RFC2256: top of the superclass chain'
#       ABSTRACT
#       MUST objectClass )

After uncommenting, though, the error is:

slapadd -b "o=Company Name,c=CH" -l dump.from.2.2.6.ldif
/etc/ldap/schema/core.schema: line 366 objectclass: Duplicate 
objectClass: "2.5.6.0"
slapadd: bad configuration file!

Yet, no other schema file contains any definition for 2.5.6.0, so I 
assume that it's compiled into slapd.

This leaves me stuck, scratching my head, and extremely grateful for any 
hint on where to look further.

Thank you all

krgds /markus