https://bugs.openldap.org/show_bug.cgi?id=9310
--- Comment #3 from Quanah Gibson-Mount quanah@openldap.org --- (In reply to Peter Childs from comment #2)
2 different MDB modules has nothing to do with schema.
You need to export your cn=config schema (slapcat -n 0 -l /tmp/config.ldif), fix the resulting export to only load the MDB module that belongs with the OpenLDAP build you're running, and then re-import it with slapadd (slapadd -n 0 -l /tmp/config.ldif), etc.
Regards, Quanah