Thanks, removing the airbrakes from the ailerons was the solution.
I did some more testing by changing the index of the airbrakes input. By using a value of 4 or lower for the airbrakes the mixer still works, meaning the control surfaces are set according to the mixer and the RC input. By choosing a value of 6 or higher the problem with the right aileron
However, while testing I discovered another issue. When the FMU crashes in the stabilized mode all servos get set to their default position and cannot be controlled anymore. But if it crashes in the manual mode the servos will respond to the RC input after a crash of the FMU.