Funcup server, but fail to register

[Penguin242]

Hi all,

I'm trying to setup a funcup only server, followed many guides, but I always get the well known error:
ERROR,INVALID SERVER,CHECK YOUR PORT FORWARDING SETTINGS
LOBBY COULD NOT BE RACHED, SHUTTING SERVER DOWN

Port forwarding IS working, I'm running on a linux system and I see traffic coming in on port 9600 UDP:
tcpdump port 9600 and '(tcp-syn|tcp-ack)!=0'
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on enp0s25, link-type EN10MB (Ethernet), snapshot length 262144 bytes
18:36:44.046288 IP 93-57-10-21.ip162.fastwebnet.it.63838 > 10.252.19.6.9600: UDP, length 4
18:36:50.645629 IP 93-57-10-21.ip162.fastwebnet.it.63849 > 10.252.19.6.9600: UDP, length 4
18:36:57.256816 IP 93-57-10-21.ip162.fastwebnet.it.63856 > 10.252.19.6.9600: UDP, length 4

Any tips are welcome....