Thanks for thinking with me.
From my tests i must conclude that DHCPrequests/offers do not go through an ACL automatically. You need to configure it on the Interface and RE firewall filters. There a Juniper documation page about this with configuration examples.
I configured dhcp-relay based on some manual i found somewhere. There they reccomended to configure "forward-snooped-clients all-interfaces" However, this basically broke the relay functionality by causing the offer packets to be dropped (show dhcp relay statistics, Packets dropped: No binding found counter went up)
Eventually i removed the option and set "forward-only" and everything started working.