I found the solution myself reading through https://tools.ietf.org/html/rfc4512 . The objectClass I am trying to create is STRUCTURAL whereas one of the super classes (posixAccount) is AUXILIARY. Following the RFC this is not allowed:
section 2.4.2 states
"Structural object classes cannot subclass auxiliary object classes."
and furthermore ...
Replace actually is a "add if does not exist and replace if exists".
"There is a one-to-one correlation between LDAP operations that modify the directory (add, delete, modify, and modrdn), and the types of changerecords described below ("add", "delete", "modify", and "modrdn" or "...
I think you’re running into this limitation https://tools.ietf.org/html/rfc4512#section-2.4.2
”An object or alias entry is characterised by precisely one structural object class superclass chain which has a single structural object class as the most subordinate object class.”