Ubuntu Networking :: Hostapd - Failed To Set Interface WLAN0 To Master Mode

I try to set-up an Access point using ubuntu 11.04. I am using the minimal configuration file from [URL] together with a TP-Link TL-WN422G (Atheros chipset). I've installed the linux-backports-net-natty-generic drivers. But when trying to start hostapd I always get this output:

Code:
# hostapd -dd -K test.conf
Configuration file: test.conf
Failed to set interface wlan0 to master mode.
nl80211 driver initialization failed.
wlan0: Unable to setup interface.
ELOOP: remaining socket: sock=5 eloop_data=0x9b9440 user_data=(nil) handler=0x43d980
I have also disabled the network-manager for this device by adding this to /etc/network/interfaces:

Code:
iface wlan0 inet manual
cat test.conf:

Code:
interface=wlan0
driver=nl80211
ssid=test
channel=1
lsusb says:

Code:
...
Bus 001 Device 005: ID 0cf3:1006 Atheros Communications, Inc. TP-Link TL-WN422G v2 802.11g [Atheros AR9271]
...
lsmo: .....

Code:
Linux magnet 2.6.38-8-generic #42-Ubuntu SMP Mon Apr 11 03:31:24 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux