Loiter crash // root cause?

My copter crashed in loiter mode.

  • APM1
  • 500mm motor distance
  • 1100kV motors
  • 9x4,7 props
  • 2200mAh 3s
    With previous firmware 2.7 loiter behaved absolutely stable.
    Now, at the first flight after update to 3.1 I had problems.
    First I observed pitch oscillations during forward flight in loiter mode.
    Then I applied yaw. The copter became instable. I tried to switch to stabilize, but the copter crashed. No big damage, only the landing skid.
    Can someone give me a hint, what is wrong with the copter? See log at line 10400.

Mediatek GPS?

No, I have equipped the copter later with a 3DR LEA 6H GPS.

X or + frame? If it is an X frame, I would say you lost your back right motor.

The solution is quite simple.
ESC calibration has not been complete.
Apperrently the motor output signal fell below zero baseline of ESC input during attitude control.
Consequence: Delay of power increase and stability problems.
So the advice to check Motors and ESCs was correct.
Thanks for your help,