I have the same issue but with a raspberry pi 4 model b.
Trying already since several days. With deconz or with ZHA or even have it on another RPI with Zigbee2MQTT. Just nothing works.
I have my extension cable. Conbee is plugged into USB 2 (have an SSD in the USB 3 port). Original power supply with 3.1 A
Still the same issue. it shows up, but as soon as i want to configure it, it wont be able to connect. And if i get it to connect, no Zigbee device will ever be detected. It only worked when i detached my SSD…
Im really thankful if someone can help.