Hi!
Thank you for the tips.
There were no site-to-site VPN firewall rule blocking the traffic.
However have multiple networks started to send event logs to the server without that I did a thing.
I was unable to ping the syslog server, but it is most likely blocking ICMP, since I tried to ping it from a network who worked as well.
I can see that there are going traffic towards the syslog server, so I will follow your other step and verify that the appliance can reach the server.