network/ath: on CURRENT no connections possible on ath-based AP

Running hostapd and a Atheros based AP on recent CURRENT (FreeBSD 11.0-CURRENT #8
r294329: Tue Jan 19 18:23:20 CET 2016 amd64), clients do not connect anymore to that
specific AP. They did a week or two ago (mobile phones, several notebooks).

The WiFi adaptor is this one (dmesg):

[...]
ath0: <Atheros AR938x> mem 0xf7e00000-0xf7e1ffff irq 16 at device 0.0 on pci1
ar9300_attach: calling ar9300_hw_attach
ar9300_hw_attach: calling ar9300_eeprom_attach
ar9300_flash_map: unimplemented for now
Restoring Cal data from DRAM
Restoring Cal data from EEPROM
ar9300_hw_attach: ar9300_eeprom_attach returned 0
ath0: [HT] enabling HT modes
ath0: [HT] enabling short-GI in 20MHz mode
ath0: [HT] 1 stream STBC receive enabled
ath0: [HT] 1 stream STBC transmit enabled
ath0: [HT] 3 RX streams; 3 TX streams
ath0: AR9380 mac 448.3 RF5110 phy 3276.12
ath0: 2GHz radio: 0x0000; 5GHz radio: 0x0000
[...]

and from ifconfig:

[...]
wlan0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
        ether xx:xx:xx:xx:xx:xx
        inet 192.168.2.1 netmask 0xffffff00 broadcast 192.168.2.255 
        media: IEEE 802.11 Wireless Ethernet autoselect mode 11g <hostap>
        status: running
        ssid Berghof channel 10 (2457 MHz 11g) bssid xx:xx:xx:xx:xx:xx
        regdomain ETSI country DE indoor ecm authmode WPA2/802.11i
        privacy MIXED deftxkey 2 AES-CCM 2:128-bit AES-CCM 3:128-bit
        txpower 30 scanvalid 60 protmode CTS wme burst dtimperiod 1 -dfs
        groups: wlan 
[...]

On some earlier dmesg outputs, I see the line 

ath0: AR9380 mac 448.3 RF5110 phy 2457.9

instead of the most recent

ath0: AR9380 mac 448.3 RF5110 phy 3276.12

I do not see any strange behaviour until clients fetch (successfully!) their IP via
isc-dhcp (isc-dhcp43-server-4.3.3P1_1), then they die or can not access  the network any
further.

Has there been a change recently?

Kind regards,

Oliver