Quadplane Qland, Qloiter problem

I am setting up a quadplane with pixhawk 2.4.8 on a Skywalker X5 with 4 lift motors and 1 pusher motor.

It hovers fine in Qstabilize mode. Transition from Qstabilize to Auto mode is fine and it flew the waypoints correctly and RTL. But once it is in QRTL or Qland, it twitches violently, drops, flips violently over and lands inverted. QLoiter displays the same problem. It always dives and crash inverted and today this broke a wing.The whole sequence lasts around 5 seconds.

Any suggestions regarding what might cause this is much appreciated.