Thanks, I really appreciate your help rweaver!
I was actually trying nmap (among other things) out when I saw your reply but the echo with telnet was really neat.
I can telnet from my desktop into the server and whatever I write on the desktop is echoed on the server, on the ports that are set to be used for passive transfer. However, it is not echoed back to the desktop. Neither have a firewall running and both are inside the network. Could it still be a problem with the router?
I don't have access to a computer outside the network (at least until friday or so) but before I read your reply I had tried three things.
First I used nmap and probed the passive ports on the router. The result was that all specified ports were filtered.
The second thing I did was to pop by www.grc.com
and try their shields up! The result was that the ports were closed. Still, it failed the test saying: "Unknown Protocol for this port. Unknown Application for this port".
Third thing I did was to try nmap on my external IP but it just said that no scan information was available.
As an added bonus I tried Shields up! one more time, after I had opened three ports with your nifty echo command and fired up Filezilla, pointing it to the external IP with passive connection. Shields up! now say I have four of the passive ports open, three of which I opened with the echo command plus one more. I killed the (three) echo jobs and tried once more. All ports closed. Fired up filezilla again and tried yet once more. All ports closed. So, I can't reproduce the forth open port, which is a bit unnerving.
Is this information of any use for you? Because I'm still lost.