After updating the firmware of my WLAN router (Fritz Box SL WLAN), my iBook (OS 10.3.9) stopped connecting automatically to my router, and also gave an annoying "error joining" message. This hint removed the error message, but automatic reconnect still failed.
Solution: when entering a WEP hex key in System Preferences > Network > Automatic Connect/Preferred Network, be sure to prepend 0x before the key itself (no spaces between 0x and the key).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060221121440533