OK, this is interesting. I tried the F50 AP. What is strange is that when pressing the AP "on" button, the AP sound is heard both when it is turned on, but also again and unusually when it is switched off. The F50 AP did not light up the AP button when I tried it in the RFN F8 or function. I switched to another aircraft and was able to use the AP normally.
I also changed the [Autopilot] section in the aircraft.cfg. Set up after takeoff!
[autopilot]
autopilot_available=1
flight_director_available=1
default_vertical_speed=1800.0
autothrottle_available=0
autothrottle_arming_required= 0
autothrottle_max_rpm = 99.9
autothrottle_takeoff_ga= 0
pitch_takeoff_ga=8 ;
max_pitch=20.0
max_pitch_acceleration=3.0
max_pitch_velocity_lo_alt=4.0 //2.0
max_pitch_velocity_hi_alt=3.0 //1.5
max_pitch_velocity_lo_alt_breakpoint=20000.0
max_pitch_velocity_hi_alt_breakpoint=28000.0
max_bank=30
max_bank_acceleration=2.7 //1.8
max_bank_velocity=4.0 //3.00
max_throttle_rate=0.20 //0.10
nav_proportional_control=11.00
nav_integrator_control=0.20
nav_derivative_control=0.00
nav_integrator_boundary=2.50
nav_derivative_boundary=0.00
gs_proportional_control=9.52
gs_integrator_control=0.26
gs_derivative_control=0.00
gs_integrator_boundary=0.70
gs_derivative_boundary=0.00
yaw_damper_gain = 1.0
direction_indicator=1
attitude_indicator=1