Osram Smart+ Bulbs suddenly unavailable after adding more bulbs

I’ve got some 20 zigbee devices connected via a Conbee II. Six devices are Osram Smart+ bulbs. I first bought a couple of bulbs, and they worked fine, so I bought four more. Now, none work as expected. I have completely lost contact with four, and two are visible but do not work as expected (they cannot be switched off, for instance).

I have tried to restart the system, and to reconfigure the bulbs, both to no avail. When doing the latter, the reconfiguration typically fails, see also screenshot:

I have also tried to reset the bulbs, also without effect.
Any advice much appreciated.

Edit: Now I have lost contact with all of them.