Clients disconnecting Wifi on 6GHz 802.11ax - no stable connection

We use FortiAPs 231G ( v7.4 build0734) on a Fortigate 200F ( V7.2.10)

The Wifi on 2.4Ghz and 5Ghz works without any problems. However with WIFI6 on 6Ghz the clients reconnect every few seconds, even on strong signal. Different clients with different WLAN adapters are affected.

Anyone having a suggestion or similar problems?

Error:

msg="AP sent disassociate frame to client c8:94:02:ff:f9:cd" msg="Client c8:94:02:ff:f9:cd had an IP address detected (by DHCP packets)." msg="Client c8:94:02:ff:f9:cd disconnected by WTP." msg="Client c8:94:02:ff:f9:cd no OKC match for PMKID" msg="Client c8:94:02:ff:f9:cd no OKC match for PMKID" msg="Client c8:94:02:ff:f9:cd no OKC match for PMKID" msg="DHCP ACK for IP 192.168.115.52 from server 10.1.1.20 with MAC 00:09:0f:09:01:1a for client c8:94:02:ff:f9:cd from router 192.168.115.254 on subnet 255.255.255.0 with dns 10.1.1.6, 10.1.1.20" msg="DHCP REQUEST for IP 192.168.115.52 from client c8:94:02:ff:f9:cd" msg="Client c8:94:02:ff:f9:cd authenticated." msg="AP received 4/4 message of 4-way handshake from client c8:94:02:ff:f9:cd" msg="AP sent 3/4 message of 4-way handshake to client c8:94:02:ff:f9:cd" msg="AP received 2/4 message of 4-way handshake from client c8:94:02:ff:f9:cd" msg="AP sent 1/4 message of 4-way handshake to client c8:94:02:ff:f9:cd" msg="AP sent reassociation response frame to client c8:94:02:ff:f9:cd" msg="AP received reassociation request frame from client c8:94:02:ff:f9:cd" msg="AP sent WPA3(non-SAE) authentication response frame to client c8:94:02:ff:f9:cd" msg="AP received WPA3(non-SAE) authentication request frame from client c8:94:02:ff:f9:cd" msg="AP sent disassociate frame to client c8:94:02:ff:f9:cd" msg="Client c8:94:02:ff:f9:cd had an IPv6 address detected (by IP packets)" msg="Client c8:94:02:ff:f9:cd had an IP address detected (by DHCP packets)." msg="Client c8:94:02:ff:f9:cd had an IP address detected (by DHCP packets)." msg="Client c8:94:02:ff:f9:cd disconnected by WTP." msg="DHCP ACK for IP 192.168.115.52 from server 10.1.1.20 with MAC 00:09:0f:09:01:1a for client c8:94:02:ff:f9:cd from router 192.168.115.254 on subnet 255.255.255.0 with dns 10.1.1.6, 10.1.1.20" msg="DHCP REQUEST for IP 192.168.115.52 from client c8:94:02:ff:f9:cd" msg="Client c8:94:02:ff:f9:cd authenticated."

Config SSID:

show full-configuration wireless-controller vap BFCLIENTS-6GHZ  config wireless-controller vap     edit "BFCLIENTS-6GHZ"         set fast-roaming enable         set external-fast-roaming disable         set atf-weight 20         set max-clients 0         set ssid "BFCLIENTS-6GHZ"         set broadcast-ssid enable         set security wpa3-enterprise         set pmf enable         set pmf-assoc-comeback-timeout 1         set pmf-sa-query-retry-timeout 2         set okc enable         set mbo disable         set voice-enterprise disable         set fast-bss-transition disable         set eapol-key-retries enable         set mac-username-delimiter hyphen         set mac-password-delimiter hyphen         set mac-calling-station-delimiter hyphen         set mac-called-station-delimiter hyphen         set mac-case uppercase         set radius-mac-auth disable         set auth radius         set encrypt AES         set radius-server "FortiAuth-BA"         set local-standalone disable         set local-bridging enable         set intra-vap-privacy disable         set schedule "always"         set ldpc rxtx         set high-efficiency enable         set target-wake-time enable         set port-macauth disable         set bss-color-partial enable         set nac disable         set vlanid 15         set vlan-auto disable         set dynamic-vlan disable         set multicast-rate 0         set multicast-enhance disable         set igmp-snooping disable         set dhcp-address-enforcement disable         set broadcast-suppression dhcp-up dhcp-ucast arp-known         set ipv6-rules drop-icmp6ra drop-icmp6rs drop-llmnr6 drop-icmp6mld2 drop-dhcp6s drop-dhcp6c ndp-proxy drop-ns-dad         set me-disable-thresh 32         set mu-mimo enable         set probe-resp-suppression disable         set radio-sensitivity disable         set vlan-pooling disable         set dhcp-option43-insertion enable         set dhcp-option82-insertion disable         set ptk-rekey disable         set gtk-rekey disable         set eap-reauth disable         set qos-profile ''         set hotspot20-profile ''         set access-control-list ''         set primary-wag-profile ''         set secondary-wag-profile ''         unset rates-11a         unset rates-11bg         unset rates-11n-ss12         unset rates-11n-ss34         set rates-11ac-mcs-map ''         set rates-11ax-mcs-map ''         set utm-status disable         set address-group-policy disable         set sticky-client-remove disable         unset beacon-advertising         set application-detection-engine disable         set l3-roaming disable     next end