Copter flips at second take off


I am having all kinds of trouble with my AUAV-X2 and AC 3.4.4. setup

Today I was out on a calibration flight.

First take off was good, I was only hovering to learn the hover parameter (which failed, see other thread)
After a while, I tested LAND mode, which work nicely.

Then I armed again, and at my next take off, the copter flipped
It disarmed but two of the motors kept spinning.
After cleanup (no damage) I tried taking off again, but again flipped.

I think, I can exlude a fundmental mistake, because the first flight was fine.

I would appreciate it, if somebody looked at my log file and could point me to the problem

2017-02-14 16-54-26.bin (1.9 MB)