Checked the code for MQTT.cpp and find the following (https://github.com/domoticz/domoticz/blob/master/hardware/MQTT.cpp - +- at Line 268)

...
else if (szCommand == "switchscene")
{
if (root["idx"].empty())
goto mqttinvaliddata;
if (!root["idx"].isInt64())
goto mqttinvaliddata;

idx ...

Retel,

I'm getting the exact same error. The MQTT message: {"command": "setuservariable", "idx" : 1,"name" : "windspeed" , "type" : "1" , "value": "10"}

gives the following error in the Domoticz log:

2015-10-27 18:29:35.777 MQTT: Topic: domoticz/in, Message: {"command":"setuservariable","idx":1 ...