postUpdate and sendCommand expect strings for the state parameter. With respect to these functions, this error usually means that the state parameter is not a string. It looks like blocky is not properly converting the color block value to a string for use in the function. Blocky integration is not 100% complete so it is not too surprising to find issues like this.
If you replace the color block with a text block that says 0, 100, 100 I think you will find that it works.
Hi,
maybe you are posting a color (41,100,100) to a switch (on/off) item? Please check if LEDDecke has a separate color item which should be used for changing the color.