Hello,

One of my drones has next issue:
Upos startup one of motors not spinning (no reaction on throttle)
It is usually random one (1, 2 or 4…3rd have not seen yet)
Issue could be repaired after disconnecting and reconnecting battery.

however once it has stopped rotating midair.

Is there something I could do about it? Whose fault is that? ESC or controller?
Is there any diagnostic tool?

There is rush stack installed.

This has happened after crash - freefall on ground (grass) from approx 30-50m

I will suggest you inspect the ESC to FC connectors. Ensure the crimped wire connections are solid and ensure the connectors on the FC and ESC boards are not loose.