Did you get to the bottom of this?
I have had two crashed in the same week with the same symptoms.
I takeoff with full flaps, after having climbed a bit I reduce to 50% flaps and keep climbing, later I reduce to 0% flaps and the planed goes to a nose-dive and crashes nose first from 80m height. Airspeed is good, I don’t have an airspeed sensor, but there was no wind and gps speed is high and throttle was at max (still climbing).
Log is here: https://www.dropbox.com/sh/j2iirts0eo0m8qx/AAACWq_fyMxdhdX70yQSov_3a?dl=0