Sorry, I also think it's the 4th. I flew this morning with the KP in the red (9 flights) in the city near wifi interferences, etc. I did not have any problems.
Compass warnings happen from time to time, even on the classic H.
That's not what caused the crash. At worst, your drone loses the north 1 second or two, and it returns to normal.
I hope you find an explanation. That's why after an update, we fly 2 meters from the ground, and we soliccite more and more gear (what I think you did). The battery that drops in tension brutally. it would be enough to measure it if it is still possible.
What version of the firmware did you have?