Thank You Jakub !
You're a true friend :)
The final solution works:
---------------------------------------------
[sssd]
...
default_domain_suffix =
ad.holding.com
[
domain/ad.holding.com]
ad_server =
kom-dc01.ad.holding.com,
kom-dc02.ad.holding.com
...
subdomains_provider = none
ldap_idmap_default_domain_sid = S-1-5-21-xxx-xxx-xxx
simple_allow_groups = KOM-SRV-Linux-Admins(a)ad.holding.com
---------------------------------------------