Many thanks, Chris. I was referring to this issue:
Anyway, the problem I reported was indeed not related to any issues with the binding. Strangely, inclusion via Habmin did not work in my case. I then dismounted the Qubino and managed to include it hooked up to my test bed. Maybe there was some kind interference where it was mounted before (it dropped out of the network previously, that is why I was messing with it in the first place). As said, inclusion via Habmin worked with a Flush Relay, so there is no generalized problem with that function.
In any case, we can close this case now, and many thanks for your insight, Chris. I am still very much of a newbie regarding ZWave and it is not always readily apparent where the problem is coming from (installation vs. devices/controller vs. binding). In my case, the problem may have been with the installation (bad antenna placement).