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Grounding can be an issue, but mostly I have seen that happened when the USB host was also connected to AC, never with a laptop on battery. I've only ever seen it cause a little noise in the temperature readings, not affect system stability. I suppose it's possible in this case with the extra AC components, ZCD and AC fan, that there are a few extra ground paths.

Rostarn, I am a little confused now. The issue you're describing in your last post is a different issue than before, am I understanding that correctly? Could we take a step back and go throught it all in more detail?

Before, with both the MOSFET and voltage regulator in place, how exactly did instability manifest? Did the Arduino reset randomly, or something else? Did you try desoldering only the voltage regulator, but leaving the MOSFET in place, and how did that affect things?

When you had the plain TC4, did you control the DC load with that too? If so, how? And were there any problems with that?

With the new problem, are you saying you are not using the DC driver at all now, as in, nothing connected to DC IN or DC OUT on the TC4+? Or that you desoldered the MOSFET completely? Or did you mean the voltage regulator?

The error message sounds like Artisan is getting some unexpected response from the Arduino when it sends a READ command. Try enabling the serial log by going to Help -> Serial, ticking serial log on, and keep that window open. Once the issue occurs, what's written in that window?