So, the issue above (created on Github by Thomas) is about the Indicator class, and the ability to set the ZHC5010’s LEDs like switches.
How about the original issue with the physical buttons? For me, the problem persists, that no matter which physical button I press, it is always the Button 0 that is switched in openHAB. I believe that is because the association groups are not properly set for the physical buttons (1-4), and thus always the relay button 0 is receiving the command.
The other way around, there is no problem. Setting switch 1-4 on/off via openHAB triggers the correct button on the physical device. Therefore I suspect that setting the association groups via HABmin, is not setting the multichannel association correctly, which is needed.