Thanks for the feedback Ken.
No, it is not thread safe. Actually I’m just vaguely familiar with these multithreading issues, having just recently started programming again when writing the AF logviewer (20 years after my university courses! :). As I said the driver is not pro at all! I quickly wrote it about 2 years ago, and it worked flawlessly until now. So I never went back to finish my quick and dirty solutions. But I never considered multithreading anyway. I just saw this old post that will surely help me with that:
Still, I’m not excluding a hardware error, the Arduino controlling the focuser was exposed to quite low temperatures that night (below 0 Celsius). Is there any reason to believe that SGP 3.1 would be more demanding on the driver in terms on handling multithread calls to it? Because it worked without a single hiccup for 2 years up to when I started using the 3.1 betas. Just trying to increase the likelihood that my repair fixes something actually broken!