Thank you @malevolent for this remark.
I had a long time now with ConBeeII in a “not connected” state. Referring to /dev/ttyACM0 gave me the idea:
I am running HOAS in a QEMU VM. I always redirected the USB-Host of the ConBee stick to the VM. After now forwarding “just” the serial device, it works like a charm including even a FW update through the web interface.
I found some additional help here: https://community.home-assistant.io/t/conbee-ii-and-kvm/333560/2 which resulted for me in a similar solution. This is even detected in HOAS and I configure the Integration and automagically all devices show up.
I hope this helps someone else, as I found plenty of disillusioned posts everywhere that ConBeeII is not working for them.