Remove ObjectClass which doesn't exist anymore in schema from user

I need to remove an object class of an old App form a few users which (the App) I removed some time ago.

what I tried to do:

ldapmodify -v -D "cn=admin,$(ucr get ldap/base)" -y /etc/ldap.secret <<__LDIF__
dn: uid=my.username,cn=users,dc=mydomain,dc=mytld
changetype: modify
delete: objectClass
objectClass: guacamoleUser

this results in the error:

ldap_modify: Invalid syntax (21)
        additional info: objectClass: value #0 invalid per syntax

I tried to do this with LDAPAdmin also, “delete row” results in the same error.

just a general question… why do we always use objectclasses instead of groups? wouldn’t groups be easier to administrate?

I just noted all the users settings and his uniquer IDs… then I re-created the user and replaced the unique IDs… till now everything works…