Network setup assistant doesn't recognize password stored in keychain

iBook OS 10.3.9 with Airport 9.52 (as shown in System Profile - says nothing about Express or Extreme, just "Airport Card", Firmware Version 9.52).

I want it to join the wifi network in our house through which four other computers happily access the internet. In Network Assistant I input the network name under "By default, join A specific network", and I enter the password that we all use, including visitors who have no trouble logging on. That password is stored and displayed in the Keychain app. But Network Setup Assistant won't make the connection, and every attempt to set it up with network name and password - going down the entire list from WEP password and the others, results in "“There was an error connecting Airport network Marimba”" or "“Sorry - the password you entered is invalid”. Anyone?