Could it be the ldap schema in use?
According to its manual, Nethserver expects a Remote LDAP server with RFC2307 schema.
While lotus domino / IBM Domino provides:
The Domino LDAP schema
The default Domino LDAP schema includes:
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Domino-specific schema elements defined by the default forms in the Domino Directory
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All LDAP-standard schema elements defined in RFCs 2252, 2256, 2798, 2247, and 2739. The LDAP service uses the file LSCHEMA.LDIF to build these elements in the default schema.