Crash Log analysis and problem diagnosis (potential bug?)

Hi all,
unfortunately few days ago my quad crashed :frowning: And it was very strange, copter was flying great and suddenly motors dropped power and it fallen like a brick.
My hardware is original pixhawk with latest ardu copter firmware.
That was not the first flight, it had more than 10h in air totally.
From mine and my friends analysis we excluded battery and power rail fail, and we think that the problem is in software bug of some kind that causes the EKF to go nuts, then software triggers crash detection and disarms the motors
Please take look on attached logs (bin and converted log)
also here is arduplotter link