When I want to update before upgrading, the
apt-get update command keeps on going. It appears that it never
stops... This happens when I do it manually, or when the update
manager is doing it automatically. Below I am showing you the output
of the update after waiting more or less 1 minute. At the bottom you
can see my sources.list.

The weird thing is, that this only happens when I try to update
through WLAN. When I connect my laptop to the wired LAN, everything
goes fine. The other weird thing is, that through WLAN I *am* able to
perform apt-get upgrade and I am able to install things with apt-get
install.

Here are the specs:
Inspiron 1501
dell_wireless_1390_wlan_minicard
ndiswrapper-1.49 (bcmwl5.inf)

I first thought it might have something to do with the firewall/proxy
(P3 w/ IPCOP 1.4.13.). But my desktop and the other wired-connected
computers all work fine behind it. My desktop has Xubuntu Gutsy and my
server uses Debian. Both work fine when executing apt-get update.
Also, my laptop encounters the same problem when I am connected to the
WLAN of the university (which does not have IPCOP...).

I have tried to change my sources.list to another country (changing nl
into be), but that didn't solve the problem. I don't know whether this
is an apt problem, or a WLAN problem, or ... So I hope some of you
know how to help me

What I now have to do is, every time the update manager shows its
orange star in the system tray, I have to kill apt-get update, connect
my laptop to a UTP cable, perform apt-get update. Than, if I want, I
can go back to WLAN and perform apt-get upgrade ...