Unwanted scan every 60 seconds

[alberto-g at inventati.org]

Hello,
I successfully configured hostapd 2.3  on a Raspberry Pi, kernel 
4.4.43-v7+.
I need 802.11a, the wifi device is a TP-Link T4U, with driver from 
https://github.com/abperiasamy/rtl8812AU_8821AU_linux.
It mostly works, but every exactly 60 seconds the clients stops 
communicating at all for about 5 seconds, then they resume normal 
operation until the next 60 seconds expires.

Output from hostapd, it repeats at each event:

nl80211: Event message available
nl80211: Drv Event 33 (NL80211_CMD_TRIGGER_SCAN) received for wlan1
wlan1: nl80211: Scan trigger
wlan1: Event SCAN_STARTED (49) received
Unknown event 49
RTM_NEWLINK: ifi_index=4 ifname=wlan1 wext ifi_family=0 
ifi_flags=0x11043 ([UP][RUNNING][LOWER_UP])
nl80211: Event message available
nl80211: Drv Event 34 (NL80211_CMD_NEW_SCAN_RESULTS) received for wlan1
wlan1: nl80211: New scan results available
nl80211: Scan probed for SSID ''
nl80211: Scan included frequencies: 2412 2417 2422 2427 2432 2437 2442 
2447 2452 2457 2462 2467 2472 5180 5200 5220 5240 5260 5280 5300 5320 
5500 5520 5540 5560 5580 5600 5620 5640 5660 5680 5700 5745 5765 5785 
5805 5825
wlan1: Event SCAN_RESULTS (3) received
VLAN: vlan_newlink(wlan1)
nl80211: Event message available
nl80211: Drv Event 19 (NL80211_CMD_NEW_STATION) received for wlan1
nl80211: New station 84:3a:4b:8f:ae:b0
nl80211: Assoc Req IEs - hexdump(len=26): 00 0e 6d 79 73 73 69 64 2d 2d 
2d 2d 35 47 68 7a 01 08 8c 12 98 24 b0 48 60 6c
wlan1: Event ASSOC (0) received
wlan1: STA 84:3a:4b:8f:ae:b0 IEEE 802.11: associated
STA did not include WPS/RSN/WPA IE in (Re)AssocReq
nl80211: Set STA flags - ifname=wlan1 addr=84:3a:4b:8f:ae:b0 
total_flags=0x1 flags_or=0x1 flags_and=0xfffffff1 authorized=1
wlan1: STA 84:3a:4b:8f:ae:b0 RADIUS: starting accounting session 
58824984-00000001
IEEE 802.1X: Ignore STA - 802.1X not enabled or forced for WPS
hostapd_new_assoc_sta: reschedule ap_handle_timer timeout for 
84:3a:4b:8f:ae:b0 (300 seconds - ap_max_inactivity)

Can anybody help me with this issue?
Thank you.

Hostapd configuration, encryption doesn't make any difference:

interface=wlan1
ssid=myssid----5Ghz
country_code=CH
ieee80211d=1
hw_mode=a
channel=40
macaddr_acl=0
auth_algs=3
wmm_enabled=0
ieee80211ac=1
vht_capab=[SHORT-GI-80][SHORT-GI-160][VHT160-80PLUS80][MU-BEAMFORMER][MU-BEAMFORMEE][SU-BEAMFORMER][SU-BEAMFORMEE]
require_vht=0
#vht_oper_chwidth=1
vht_oper_centr_freq_seg0_idx=40
#vht_oper_centr_freq_seg1_idx=44

#wpa=2
#wpa_passphrase=12345678
#wpa_key_mgmt=WPA-PSK WPA-PSK-SHA256
#wpa_pairwise=CCMP
#rsn_pairwise=CCMP