Problem with an FTP server

Greetings,
I have a problem with an FTP server I'm trying to run on my PC.
I have installed xlight FTP and I have set it up so that it gets a dynamic IP from my router. On my router I have opened port 21 and I have configured it so that it port forwards to my PC's IP. I have disabled the security service from my ISP. Also I have configured my firewall properly.
When I connect to my FTP server from a local host, everything works great. But I can't connect from a host outside my LAN (through the Internet). Any ideas what may cause this?
