Thanks Rene. The button works well for exclusion; just long-press for 5 secs or so, per the instructions. Just as hammering for inclusion, it doesn't work every time but after two or three.
But same issues as before sadly. Damn. 
EDIT:
I just want to add that the issues has been found; Hubitat are using older and buggy firmware for their EU Z-Wave sticks, and very likely for the EU C-7 as well. Some more details here.
FYI @MrFarmer: Using the updated firmware there are zero issues with S0 security as well. At least on Home Assistant.
