Engine loss due to wrong PID tuning?


I built a Quadcopter drone and used the setup guid at the ardupilot website to set it up. The drone is flying quite well unfortunately it seems like one engine just cut out, which causes the drone to flip and crash. According to the logs it seems the Pixhawk just throttle down one engine.
Pleas let me know how I can tackle this problem. Or does one of you had the same issue?