Thanks a lot, did some more reading and I assume I got it (at least high level). For me the discussion here ZWave - Lifeline vs Controller updates vs Device status was quite helpful to understand whats going on.
In short: With ZwavePlus devices (like the Fibaro CO Sensor), the so called “Lifeline” accociation group (with ID 1 according ZwavePlus spec) is responsible to send (ALL) updates to the controller. In this Lifeline group the controller (node_1) must be listed so OpenHAB receives ALL updates from the Device.
This is the case in PaperUI (Things -> MyCOSensor1 -> Edit -> Association Groups and I still have node_1 in all the other association groups. I will remove node_1 again from all Association groups and will only keep Group 1 (Lifeline) assigned to node_1 and will give it a try again (will check if I still receive the alerts and CO values) as recommended by you.
What confused me was that in HABmin all the accociation groups are empty, in PaperUI they are still populated. Maybe an issue with HABmin?
Anyway I will ignore HABmin as I assume its more or less not maintained anymore and will focus on PaperUI.
Regards,
Thomas
PS: I use OH 2.4 stable