REST endpoint /organizationalunits/

{organizationalUnitName} with method POST is used for updating an existing organizational unit. REST method PUT should be used for updating. POST method is for creating.
REST endpoint is implemented here

Also REST endpoint /organizationalunits with method POST is used for creating new organizational unit. This endpoint should be change to /organizationalunits/{organizationalUnitName}

where

{organizationalUnitName}

is same as org. unit name in REST call body.