I downloaded MQTTfx in order to test the same configuration you have. I want to ensure I am executing the same steps you did to receive this error. Let’s begin on Losant’s side.
I have a device, MQTT Device, that I have created. I first copy the device ID to use as the “Client ID” in MQTTfx:
Next, I create an Access Key in Losant:
And include the credentials provided as the “Username” and “Password” in MQTTfx:
Finally, when I click “Connect,” in MQTTfx, I can see on my device page that I have successfully connected to Losant:
Below are my logs in MQTTfx:
I am curious, is this the same process you followed? Does the “Connect” button fail to work, and result in the logs you have included?