Server actively denying connections
Did you set your server instance to Deny and make sure there are no host entries listed? Good. If not, read about the ServerAdministration, Configuring_ATCSMon_as_a_server Deny server connections function.
Just no connections at all
The previous mention of the Deny checkbox is the most common way folks get confounded. Also check your DNS or DynDNS setup, make sure your private LAN address for the server box didn't change, and that your router has the TCP port redirected to your private IP correctly, and the UDP range you specified in the application is redirected there as well (optional, recommended...read on).
Verifying TCP connectivity
If unable to connect, the first part of the connection negotiation involves a TCP connection to the server from a client. We can emulate that behavior by a simple test. From the client PC, go to Start->Run and type in CMD and enter to open a command prompt. From the command prompt type telnet (servername) (serverport). For example, telnet atcsserver.gotdns.com 4800. All you should see is the screen will clear and display a number, probably a 5-digit number. If you don't get that, there's something basic wrong, most likely with one of; the ATCS Monitor configuration, a software firewall, or the port forwarding configuration in the server broadband router. Go back to the section on section on Server Administration to see if that helps you.
Some connections, but others failing with "No UDP ports available"
...or some message like that. You need to configure the "Base=xxxxx,yy" option to match the routers redirected UDP range. Re-read the section on ServerAdministration if this doesn't sound familiar.
-- Main.GaryHahn - 31 Aug 2008