Network manager makes computer hang
 

Ubuntu Hardy is my first experience with Linux, so I am a real newbie. While starting many times the (wireless) network manager nm-applet makes my computer hang, mouse and keys are dead, so I have to perform a new boot. The second or third try the connection succeeds. While logging off I see on the black screen mentions like assertion databus connections failed (too less time to read it exactly), maybe this has to do with it?

This seems strange, how could you specially determine that the network manager applet is the cause of your startup failure?
If so, try to disable the network manager applet from starting up and make sure that it really the cause of the problem.

You can disable the applet from start like this from the menus:
system >> preferences >> sessions
and remove the tick beside the network manager and reboot.

could be a compatibility issue also check that your card is good

Thanks to khalisafwat and tommytomthms5! I disabled the network manager and rebooted five times, no problem occured. Of course I have to enable it again to test if the problem appeared again, but I - stupid - changed setttings in the settings box. First I lost connection, but just before I wanted to restore a backup the connection came back. So I still don´t know what happened.
Strange enough in het Settings box only WIRED connection is checked (Address: dhcp) and WIRELESS Connection (roaming mode enabled) has a minus-sign. Is this normal for a well-functioning wireless connection? I don´t see the logic of it. Since then I got no problems with the connection. Maybe again after I post this ;-) I cross fingers.

Problem seems solved
 

By accident I discovered that the wireless connection worked fine when I moved my usb-adapter away from the receiver of my... Logitec wireless keyboeard and mouse! These two seem to interfere and this explains why I couldn´t use my mouse and keyboard after starting up nm-applet...