I have to reboot in order to connect (!)

I use NetworkManager, ndiswrapper and kde 4.8 rc on 11.4

When booting and entering the plasma desktop, I am connected to wi-fi.
If I disconnect, I can’t re-connect. NetworkManager keeps asking me the password.

If I just reboot, It connects without problems.

Thank you in advance.

You should substitute k-networkmanager with nm-applet gnome. This software (k-networkmanager applet) is beta level.
You can relaunch your networksystem and networkmanager without rebooting.
Open a terminal and type:

su -
rcnetwork restart

wait that you get back to the promt.
then:

exit

so you won’t leave a root session open in shell. It’s a good habit to exit.
Now it should work again. If your network card is working well, and you do not experience problems during connect, chances are good that the culprit is (as usual) the k-networkmanager applet.

rcnetwork restart

That worked, thank you.

The network I connect to Is from a cafe near my house and the signal is very, very week. I don’t know what to blame.

The fact that an emitter is no strong enough should not cause the network server to crash. You can try to go near to the café and if with the stronger signal you have the same problem, the culprit is not the signal. The nm-applet is easy to install and to substitute for the kde-pendant. You may try via yast. There is a quasi how-to on this site, use the search function (“I gave up on networkmanager”. Should you not find it, I will post the link here. Regards BTW, I forgot: a restart is only seldom necessary. The services are often organized in servers so e.g. if the sound system hangs, you can do the same as before with:

su -
rcalsasound restart
exit

There is always something to be done to restart a service without rebooting.