Error for wireless request "Set Mode" (8B06)

Hi all,

in mode "Managed" my atmel-usb-802.11b (P:  Vendor=1371 ProdID=5743
Rev=1.00, berlios driver: Wireless LAN Driver v0.12beta14, running on
kernel 2.6.6, regulatory domain FCC (U.S) (id 16)) works fine.

However if I do want to set mode "Master" it fails:
keeper:~# iwconfig wlan0 mode Master
Error for wireless request "Set Mode" (8B06) :
    SET failed on device wlan0 ; Invalid argument.


Below you will find additional infos like
- iwconfig
- syslog
- kernel
- wirelesstools-version


wireless-tools
##############
keeper:~/at76c503a# dpkg -l | grep wirel
ii  atmel-firmware 1.0-2          Firmware for Atmel at76c50x wireless
network
ii  wireless-tools 26+27pre22-1   Tools for manipulating Linux Wireless
Extens


iwconfig
#########

keeper:~/at76c503a# iwconfig wlan0 mode managed channel 11 essid any
keeper:~/at76c503a# ifconfig wlan0 up
keeper:~/at76c503a# iwconfig wlan0
wlan0     IEEE 802.11-DS  ESSID:""  Nickname:"okuwlan"
          Mode:Managed  Channel:11  Access Point: 00:0D:54:A7:D2:A2
          Bit Rate:11Mb/s   Tx-Power=15 dBm
          Retry limit:8   RTS thr=1536 B   Fragment thr=1536 B
          Encryption key:off
          Power Management:off
          Link Quality=0/0  Signal level=-124 dBm  Noise level=-256 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0


syslog
######

Jul 30 18:32:59 keeper kernel: usb 1-2: new full speed USB device using
address 5
Jul 30 18:33:00 keeper kernel: /root/at76c503a/at76c503-fw_skel.c: Atmel
at76c503 (RFMD) Wireless LAN Driver v0.12beta14 loading
Jul 30 18:33:00 keeper kernel: /root/at76c503a/at76c503-fw_skel.c: using
compiled-in firmware
Jul 30 18:33:00 keeper usb.agent[2644]:      at76c503-rfmd: loaded
sucessfully
Jul 30 18:33:00 keeper kernel: usbcore: registered new driver
at76c503-rfmd
Jul 30 18:33:02 keeper kernel: /root/at76c503a/at76c503.c: $Id:
at76c503.c,v 1.60 2004/06/15 18:20:37 jal2 Exp $ compiled Jul 30 2004
07:55:40
Jul 30 18:33:02 keeper kernel: /root/at76c503a/at76c503.c: firmware
version 1.101.2 #84 (fcs_len 4)
Jul 30 18:33:02 keeper kernel: /root/at76c503a/at76c503.c: device's MAC
00:08:a1:42:3b:1b, regulatory domain FCC (U.S) (id 16)
Jul 30 18:33:02 keeper kernel: /root/at76c503a/at76c503.c: registered
wlan0


modprobe -v 
############
keeper:~# modprobe -v at76c503-rfmd-acc
insmod /lib/modules/2.6.6/kernel/drivers/usb/usbdfu.ko
insmod /lib/modules/2.6.6/kernel/drivers/usb/at76c503.ko
insmod /lib/modules/2.6.6/kernel/drivers/usb/at76c503-rfmd-acc.ko


Any idea whats going wrong? 

Thanks

christian