Okay i found a workaround. i have IPv4 and IPv6 enabled and DNS entrys for all APs. Default my client try to access the AP over IPv6 first, this worked fine on 1.16 but seems to be broken on 1.20. Accessing WebGUI over IPv6 now gives me error 403 access denied.
Workaround is to access the Web Interface with IPv4 Address. SSH still works with both protocols.