I am going to assume you're using a windows server, since I didn't hear you mention it (and I really don't know much about the linux setup).
If they're getting kicked while zoning, check a couple things:
- make sure you have enough zone.exe instances running.
- make sure that you have your firewall set up correctly. By default, your zones start at port 7000 and increment from there. Ensure that you are port forwarding tcp and udp 7000 - 7100 or whatever to your server.
It sounds to me like an issue with your firewall ports, personally.
Dax