Try running on a standard map, and see if multiple people can connect, and we'll go from there. Better yet, leave an IP address so we can try connecting after you do so.
Also, what is your setting in soldat.ini for lobby forwarding? Your firewall might allow Soldat to access the outside net, but block clients from connecting to Soldat itself, since it has no way of knowing that random connection request X is meant for soldat. The first client might be allowed to connect because it happens to use port 23073, which is probably forwarded to Soldat, but the next player can't.
I've seen this with some models of routers, they will allow the first connection then cut off the rest. Usually the result of some misconfiguration of the settings.
