I don't use Fedora but I would think this works there as it does on ubuntu/debian

Open your terminal and type

lsof -i :30003

that command will list you the application used by that port with PID and also a userID if thats helpful

for example

Also, other things to look at are:

what does ifconfig tell you about the server where x2goserver is running?  what IP address is it?

Does the machine have more than 1 network interface?   what are the IP addr of each?

Are there any virtual machines running on that server?   what are their IP addresses

What's the IP address(s) on your client, is it running any virtual machines?

What does traceroute tell you about the path to 225.3.10.80

Brian