Thanks @zarthan for suggesting to work on the xml configuration file, now I finally got it to work. I had a look at the xml configuration of the device that triggered the upgrade of the command class and it contained the key to success for setting up Thermostat Setpoint:
<CommandClass id="67" base="0" typeInterpretation="A" />
Suddenly the correct temperature settings were read from the device and changing the temperature actually works now… The key was typeInterpretation=“A” that refers to the Z-Wave public Command class specifications for Thermostat setpoint v2 and v3. Need a bit more work on the configuration file, but at least I’ve established contact…