hostapd 0.7.3. : problem : deauthenticated due to inactivity

[ph35sm at free.fr]

Hi,

Using Ubuntu Server 12.04 LTS with hostapd (v0.7.3) since more 50 days without any problem.
Server rebooted this morning after packages updated.

Now my wifi devices (thinkpad, android phones ...) cannot get connected. 

Hereafter are the syslog messages : 
Sep  7 09:37:19 x hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: authenticated
Sep  7 09:37:19 x hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: associated (aid 1)
Sep  7 09:37:19 x hostapd: wlan0: STA 78:d6:f0:2f:51:ee RADIUS: starting accounting session 5049A416-00000000
Sep  7 09:37:20 x dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:20 x dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:23 x dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:23 x dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:30 x dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:30 x dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:47 x dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:47 x dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:51 x dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:51 x dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:51 x hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: disassociated
Sep  7 09:37:52 x hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: Sep  7 09:37:19 surfsm hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: authenticated
Sep  7 09:37:19 surfsm hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: associated (aid 1)
Sep  7 09:37:19 surfsm hostapd: wlan0: STA 78:d6:f0:2f:51:ee RADIUS: starting accounting session 5049A416-00000000
Sep  7 09:37:20 surfsm dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:20 surfsm dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:23 surfsm dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:23 surfsm dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:30 surfsm dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:30 surfsm dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:47 surfsm dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:47 surfsm dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:51 surfsm dhcpd: DHCPDISCOVER from 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:51 surfsm dhcpd: DHCPOFFER on 10.1.1.21 to 78:d6:f0:2f:51:ee via wlan0
Sep  7 09:37:51 surfsm hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: disassociated
Sep  7 09:37:52 surfsm hostapd: wlan0: STA 78:d6:f0:2f:51:ee IEEE 802.11: deauthenticated due to inactivity


/etc/hostapd.conf :
interface=wlan0
driver=nl80211
ssid=xxxxx
hw_mode=g
channel=9
ieee80211n=1
max_num_sta=10
macaddr_acl=0
dynamic_vlan=0
dump_file=/tmp/hostapd.dump
ctrl_interface=/var/run/hostapd
ctrl_interface_group=0
country_code=FR
own_ip_addr=10.1.1.254

The AP is open, no WEP.

I make some search on web .. but nothing positive.
I think the problem is "deauthenticated due to inactivity" 

Is anybody have a solution ?

Many many thanks
Phil.