Store data with an AD user in LDAP

I'm migrating from samba3+ldap to a samba4 domain (with samba 4.7.6-Ubuntu). Part of this migration has to be the storage of some data our services need in the user objects. (Nextcloud quota, alternative mail addresses, ...)

I've added my own schema (bssUser) with special attributes (bssNextcloudQuota, ...) to the AD and can see them in the schema editor. All attributes are optional in bssUser (MAY in the schema LDIF). When I add the following ldif to the AD, I cannot create new users in the Domain anymore. When I revert the change, everything is fine again.

dn: CN=User,CN=Schema,CN=Configuration,DC=bss,DC=mydomain,DC=de
changetype: modify
add: auxiliaryClass
auxiliaryClass: bssAccount

Am I going at this wrong? Whats the best practice to save some data in AD with the user?