I found out that with entering a DMZ (demiliatrized zone) IP-adres I can get it to work. Of couse, this has two drawbacks :
1) I have to reboot the PC/router each time I enter this value. So why not leave it you say? Well, a DMZ-ip
2) totally exposes the PC to which that IP belongs. So of couse MSN works, but a pretty high price.

For now, if I want to send a file to one of my contacts I'll use the DMZ option. But I'll keep on searching for another solution.