Really curious. I'm surprised that the diagnostic codes couldn't be read and that the STC is a different unit in any way other than some euro certification. I've scoured the STC and ST manuals side by side and can't see any differences. Must just be firmware differences. Physically they appear to be the same--spare parts are often labeled as fitting both the ST and STC (burner, glow plug, etc.). It certainly is possible that ethanol is disliked by a European tuned model, but that would be surprising. That said, what do they sell in Brazil? Maybe the STC? Good luck.
PS> It might be worth checking if maybe the heater came set for high altitude. Doubtful, but toggling to the other mode might change things. It's a hail Mary for sure, but might be worth a try. They say that once set for altitude you can leave it there--even at sea level--with the only side effects being a leaner burn (cleaner burner over time) and a loss of 3-5% max heat output. Might be worth fiddling with it as any change in mixture might make the heater fire differently and more promptly.