Cannot Connect new Devices to conbee II Stick

Partially yes, but not if want to use ZHA or Zigbee2MQTT instead of deCONZ. Personally I prefer ZHA and Zigbee2MQTT.

Regardless it does currently not make sense to buy any ConBee dongle if want to use ZHA or Zigbee2MQTT since they thier support for ConBee is not as good as their support for dongles Silabs’s EmberZNet/Ember and TI’s Z-Stack/ZNP firmware

So if a user was successfully using ZHA or Zigbee2MQTT with a ConBee II (ConBee 2) then they should not buy a ConBee III (ConBee 3) as an upgrade if want to continue using ZHA or Zigbee2MQTT. They should instead buy a new dongle with Silabs’s EmberZNet/Ember and TI’s Z-Stack/ZNP firmware (e.g. SkyConnect ZBT-1 or a CC2652P based adapter).

You should really only buy a ConBee if want option to use deCONZ as when using ZHA or Zigbee2MQTT then ConBee dongles have limitations that EmberZNet and Z-Stack firmware does not have when using those, (install code and QR code commissinibg does not work on ConBee with ZHA or Z2M, nor does backup and restore work in Zigbee2MQTT).

All that is explained more here:

Also read this for technical details: