Error "zh:deconz:driver: no free slots. Delay sending of APS Request"

Folks,

This error started to appear a couple of days ago. It randomly shows up after HA restart and I need to repeat it until it goes away. Eventually it does and the Conbee II stick works again. Any idea what I can do to fix it?

Error message repeating in the logs: zh:deconz:driver: no free slots. Delay sending of APS Request

Thanks

Pattern discovered:
restart of the HA or the RPI causes the error
physical shutdown, power off, power on & restart: avoids the error

Any ideas?