WindowsServer 2008 not accepting connections to serverprograms

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http://serverfault.com – A company which our agency works for got a Windows Server 2008. I've had the job to install an FTP-Server (FileZilla Server) on this Windows Server 2008. Now my problem is that the server does not accept most of the connections. VPN-Connections to this server work [even from outside], but ftp from outside the local network does not work. I've also written a little tool to test which connections the server accepts - it accepts connections from inside the network but not from outside the network. The router is properly configured, the packets are even visible in wireshark on that server, but t (HowTos)