Ardurover (boat) - change flight mode error and error 0x800

Hi everybody!
I made a boat (with two forward motors) and a pixhawk 2.4.8 (ArduRover V4.0.0).
Everything seems to work with motors responding to throttle and steering as expected.
For some reason though I cannot change “flight mode” as I always get error (for AUTO and for RTL).
I suspect it is due to the altitude (I guess I read something about this in an error on Qgroundcontrol on mobile, but not sure where…).
Here is the link to the bin file for your convenience of a test of flight mode changing .

Furthermore, recently I get also an “internal error 0x800” and I cannot understand what is causing it. It may be unrelated, but it happened after that previous mission (no bin file for that yet).

Any help is appreciated.