Just some extra info; looking at the traffic with wireshark; i can see that the HS sends the right messages with destination 126.96.36.199 to port 3671. See packet below.
501155 946.852983 192.168.178.122 188.8.131.52 KNXnetIP 60 ROUTING_INDICATION 3671 > 3671| cEMI: L_Data.ind
The contents of the packet show the right physical adress etc and the right source adress.
Somehow the packet is not put on the KNX network bt one of the gateways…