FortiAP's 221C - after about 1 day i can't connect to 5Ghz, only reboot of alle AP's helps

Hello all, 

hat problems with my 9/10 year old Fortigateconfig, so i reconfigured everything new on 6.4.3 per hand. Since the new setup, sometimes i'am not able to connect to 5Ghz on my two 221C anymore. Only a reboot, disable enable 5Ghz/SSID helps. The strange thing is i see the SSID and if connect sometimes the connection works, but with really really poor signal in log of the fortigate. But on the client side it shows me a really good/best signal. 

This day on 08:55 AM all Clients where disconnced. Before that it ran for a few days without any problems. That has been the case a few times now. An update to 6.4.4 didn't help either. FortiAPs are on the last available firmwareversion.

here are my AP Configs: 

config wireless-controller wtp-profile
    edit "FAP-221C"
        config platform
            set type 221C
        end
        set handoff-sta-thresh 30
        set allowaccess https ssh snmp
        set frequency-handoff enable
        set ap-handoff enable
        config radio-1
            set band 802.11n-only
            set short-guard-interval enable
            set auto-power-level enable
            set wids-profile "osit"
            set darrp enable
            set vap-all manual
            set vaps "bodyguide-dev" "surfen"
            set vap1 "surfen"
            set vap2 "bodyguide-dev"
            set channel "1" "6" "11"
        end
        config radio-2
            set band 802.11ac-only
            set short-guard-interval enable
            set channel-bonding 80MHz
            set auto-power-level enable
            set wids-profile "osit"
            set darrp enable
            set vap-all manual
            set vaps "wlan-bodyguide" "bodyguidemobile" "surfen"
            set vap1 "surfen"
            set vap2 "wlan-bodyguide"
            set vap3 "bodyguidemobile"
            set channel "36" "40" "44" "48" "52" "56" "60" "64" "100" "104" "108" "112" "116" "120" "124" "128"
        end
    next
end

config wireless-controller ble-profile
edit "fortiap-discovery"
set advertising ibeacon eddystone-uid eddystone-url
set ibeacon-uuid "wtp-uuid"
next
end

config wireless-controller wids-profile
    edit "default-wids-apscan-enabled"
        set ap-scan enable
    next
    edit "my-special-default"
        set ap-scan enable
        set wireless-bridge enable
        set deauth-broadcast enable
        set null-ssid-probe-resp enable
        set long-duration-attack enable
        set invalid-mac-oui enable
        set weak-wep-iv enable
        set auth-frame-flood enable
        set assoc-frame-flood enable
        set spoofed-deauth enable
        set asleap-attack enable
        set eapol-start-flood enable
        set eapol-logoff-flood enable
        set eapol-succ-flood enable
        set eapol-fail-flood enable
        set eapol-pre-succ-flood enable
        set eapol-pre-fail-flood enable
    next
end

It doesn't depend on the SSID. Then I move "surfing" to 5Ghz only, then it doesn't work anymore. So it only affects 5Ghz.

Very thanks and best Regards