Hello.

I was trying to setup an upnp server. And it kept complaining that the port is not available. Eventually I found out that my kde installation keeps listening on port 1900, which that server tries to use. So, can someone tell me how to disable this behavior?

Here's the output of netstat -tulpn:
udp 0 0 0.0.0.0:1900 0.0.0.0:* 981/kdeinit4: krunn
udp 0 0 0.0.0.0:1900 0.0.0.0:* 998/knotify4
udp 0 0 0.0.0.0:1900 0.0.0.0:* 890/kdeinit4: plasm
udp 0 0 0.0.0.0:1900 0.0.0.0:* 832/kdeinit4: kded4

KDE version is 4.14.

They're not "listening" but subscribed to the same multicast address (likely for UPnP events) - that's no problem, UDP isn't TCP.
=> I assume you're trying to bind some TCP port of your UPnP server to 1900? (In case: that's wrong)