It is Golden Motor battery. The leds are burning the whole time in the throttle, plus there is a sound from the motor (if you roll the wheel manually and trigger throttle) when the fault appears after start.
It's like the motor is trying to trigger the coils, or part of the strength.
The wheel doesn't move even in the test bench now.
Controller seems to behave logically for the few seconds it works.
I could try to open the controller box, and measure the voltages, but I'd like to hear if you have any other suggestions.
Edit: just opened the battery to check the key switch, it looked ok. 53,3V on key side, 53,4V on charger side (battery not full).
Any test I can do? Hook the battery from charger side?
Maybe just bypass the keyswitch and solder the connector to the output.
Edit: I brought the whole system indoors for two hours, at first it was malfunctioning like normally, then used hairdryer to blow hot air into the keyhole. Now I can't get this fault to manifest itself anymore again. On the other hand, this happened yesterday too and now there is hardly any load on the motor either. Voltages are quite steady around 52,8 and 53,3V when it runs. No sudden drops, but then again no fault.
This happened yesterday as I took it inside. It's now 5C outside.
I'll take it for a spin now, let's see if something happens... putting some tape on the keyhole to prevent any drops from the road.