OpenLDAPaci not allowed in user-defined object classes

[Michael Weiser <mweiser@fachschaft.imn.htwk-leipzig.de>]

Hello,

I used to run a small openldap installation using version 2.0.21 with a
couple of user-defined object classes, one of them including an
OpenLDAPaci defined as

objectClass ( 1.3.6.1.4.1.4203.1.4.50 NAME 'htwkPerson'
        DESC 'HTWK Person'
        SUP inetOrgPerson
        MUST ( userPassword $ htwkStatus )
        MAY ( htwkMailingLists $ OpenLDAPaci ))

( I know, I've used OIDs of the OpenLDAP OID namespace but the directory's
only for internal, experimental use. )

After upgrading to openldap-2.1.3 I get the error message

/etc/openldap/schema/htwk.schema: line 42: ObjectClass
operational: "( 1.3.6.1.4.1.4203.1.4.50 NAME 'htwkPerson' DESC 'HTWK
Person' SUP inetOrgPerson MUST ( userPassword $ htwkStatus ) MAY  (
htwkMailingLists $ OpenLDAPaci ))

Using FAQs, mailing-list-archives and the web I found out that this is due
to the USAGE definition enforcement in openldap-2.1 and tried to redesign
the schema but I don't seem to get it right and I can't find any
documentation on it. Therefore I'm sorry to bother you with such an
FAQ-like question, but how do I add OpenLDAPacis to user-defined object
classes if the former are operational and therefore not allowed in the
latter?
-- 
Thanks very much in advance,
Micha