Ah very interesting...
So then as you say
if the true purpose of the A1 seems to be to return a feedback signal to the MCU, to say the motor is in fact spinning around. Because if it is not blowing any air (with a valid PWM signal)... then the MCU should probably turn off the heater. To prevent damage or overheating in that error condition. Since the heater requires an airflow to keep it cool. So this might also help to explain why there is no heating ATM. And the screen reads "0c". If it did not receive fan spin return signal.
OK so looking at what you are calling the ESC board (sorry I do not know these terminology so well). It is in fact an Allegro A4931 brushless DC fan controller chip. And so this is a great start already, since we know BLDC motors require pulsing at the right times and although I know almost nothing about bldc. Clearly there is a great potential for some high current upset or pulses / backfeeding voltage spikes etc. All on that BLDC board.
So while I still did not begin to debug the real issue (yet). I can see maybe a clue in my microscope, which is between the pin header contacts for the motor. Because between the different pins, and phases of the motor wires are very thin and long tin whiskers.
Actually I already broke them up mostly, in trying to wipe them away too soon. But clearly this is due to the lead free solder. And also the high current pulses due to BLDC motor driving. Which in combination with no protective coating etc... there was probably some sort of bridging occured. Due to these tin whiskers.
So maybe an event occured during operation, with a current from one coil got dumped to the pin for another coil. Or one of the hall sense wires. And then some large current or induced voltage spike on the wrong connection. Or at the wrong relative timings. Whatever happened. This seems the most likely reason for the failure as far as my limited knowledge permits. Because since this BLDC fan seems reasonably somewhat powerful output device, it might be enough to cause some sort of a damage.
Although these tin whiskers are really thin filament. So maybe if i just clean them up and no damage was done here. Then it might start to work again. If it is just a safety cut out feature engaged by the BLDC controller chip IDK.
But for everybody else out there with these stations.... I think you guys all should be unplugging and removing this board. And inspecting it for the tin whiskers. Then cleaning it up. Then maybe just apply some silastic, or other physical barrier underneath the long pin header pins. (that the soldered pins sticks out on the under side).
For the physical barrier to create IDK what is best. But maybe just some silastic, or glue gun. Or conformal coating IDK. Should be enough for only +15v DC. Its a low voltage.
Please take this advice because they really will grow over time. The more hours of operation for the fan and get worse. So it is worth the hassle to protect against this.