I was able to connect with wep. What i did is i created a file in /mnt/bootpar named wpa_supplicant_wep.conf
The file contains the following parametres:
Then i modified the file startwlan in /home/root/lejos/bin to use the file /mnt/bootpar/wpa_supplicant_wep.conf
instead of /mnt/bootpar/wpa_supplicant.conf.
Now everything works fine.