Mirko,
From looking at the dataflash log’s STRT (steering rate) message, the vehicle appears to be turning in the opposite direction from the way it should. Normally this is caused by a mistake in the setup of the motors (i.e. SERVOx_FUNCTION, SERVOx_REVERSED).
You’ve read the wiki’s configuration section including checking the direction of the motors and/or servos?
Also there’s an important section on tuning the steering controller here.