Mission don´t start

I have finished building my arduboat with a Pixhawk 2 so far. In manual mode, it drives very well. But the next step seems to be a bit tricky.
The mode switch is set correctly. (Manual / Auto / Guided). I can set the boat to be armed. After setting some waypoints, I switch from manual to auto. Actually, at this point it should start the mission, or? But nothing happens.
Every now and then comes the message “Bad AHRS”
Another thing, I use a Spektrum DX8 TX and a satellite receiver at the Pixhawk. The DX8 is Mode2, so throttle left, only forward. Since the thruster needs forward/backward, I mapped the right-hand stick like this:
RCIN1 (Pitch) to ServoOut 1 (throttle)
RCIN2 (Roll) to ServoOut 3 (steering)
As said, in manual mode everthing is working pretty well. But nothing happens when I switch to auto. The LEDs on the Here GPS is solid green.
Any ideas? Thanks!

can you post a .bin log?

you just set Arm check parm= to zero. And then auto mode working will be fine,

Please do not suggest people do this.

On many vehicles this is dangerous.

It is never necessary. We have ways to force the vehicle into an armed
state without making a permanent change to the vehicle configuration. In
MAVProxy this is “arm throttle force”, I believe MissionPlanner offers a
force-arm option in its interface somewhere now.

Actually I set arming_check to 0. But nothing happens.
Here comes the logfile:

I finally solved the problem. As said in my first post, I changed ServoOut1 to RCIN1 and ServoOut3 to RCIN2 (which are the two channels of the right stick on my Transmitter). But it obviously has to be groundsteering and throttle. Now I can startup a mission, but have throttle on the left stick, which is not spring-centered. To change this, I have to use RCMAP instead of ServoOut, right?

Yes, You have SERVO1 and SERVO3_FUNCTION set to 51 and 52. You need to change these functions to the proper # for the type of steering and throttle you are using. RCMAP probably need some adjusting for the Spektrum stuff.