Spring Boot: How to define new object classes in UnboundID LDAP?

Question

In a Spring Boot 2.1.6 project I'm using the embedded LDAP service (UnboundID LDAP). I need to define new attribute types and object classes. My LDIF file contains something like that:

dn: cn=schema
changetype: modify
add: attributetypes
attributetypes: ( 2.25.128424792425578037463837247958458780603.1 NAME 'customAttribute' DESC 'a custom attribute' EQUALITY caseIgnoreMatch SYNTAX '1.3.6.1.4.1.1466.115.121.1.15' )

However, when I deploy my WAR on tomcat I get this exception:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'directoryServer' defined in class path resource [org/springframework/boot/autoconfigure/ldap/embedded/EmbeddedLdapAutoConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.unboundid.ldap.listener.InMemoryDirectoryServer]: Factory method 'directoryServer' threw exception; nested exception is java.lang.IllegalStateException: Unable to load LDIF classpath:test.ldif
...
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.unboundid.ldap.listener.InMemoryDirectoryServer]: Factory method 'directoryServer' threw exception; nested exception is java.lang.IllegalStateException: Unable to load LDIF classpath:test.ldif
...
Caused by: LDAPException(resultCode=68 (entry already exists), errorMessage='Unable to add an entry with a DN that is the same as or subordinate to the subschema subentry DN 'cn=schema'.', ldapSDKVersion=4.0.11, revision=34e39aab27ea4fb92659a6888933db08099c7e41)
    at com.unboundid.ldap.listener.InMemoryRequestHandler.addEntry(InMemoryRequestHandler.java:4916)
    at com.unboundid.ldap.listener.InMemoryRequestHandler.importFromLDIF(InMemoryRequestHandler.java:4624)
    at com.unboundid.ldap.listener.InMemoryDirectoryServer.importFromLDIF(InMemoryDirectoryServer.java:1255)
    at org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration.importLdif(EmbeddedLdapAutoConfiguration.java:164)
    ... 76 more

What happens obviously is that, for some reason, the LDAP service "thinks" that I'm trying to ad the entry "cn=schema", and refuses the operation as the entry already exists, while I'm trying to modify it, hence the statememnt "changetype: modify" in the LDIF file.

Could someone please advise me what I'm doing wrong ?

Many thanks in advance.

Kind regards,

Nicolas