RB951-2n v6.19 wifi disconnects???

hello
i installed an RB951-2n to a client a week ago updating it to ROS v6.19 and creating a simple hotspot on wifi.
client complains about regular disconnects from smartphones. it has medium to full signal strength and suddenly losses the network for 3-4 secs
and reconnects again. i tested using my own phone and indeed it drops the signal as if wifi stops (no ssid) and up again. this happens approx every 20 minutes or earlier.
i did not try to downgrade to v5.26 as i am afraid of broken configuration but i may do it reconfiguring again.
any ideas or maybe a bug?

Nevermind my mistake. i need to change login setting from server profile (disable cookies and enable mac cookies for smartphones).
Mikrotik Rocks!!!