Pitch links and swashplate.
An area where even some of the most experienced manufacturers and designers have got it wrong in the past.
Look at every prototype model in recent times and look at the amount of strain gauge work that is done in this area.
I would not be surprised if this was not just a simple component failure. Very complex stuff going on here. Possibly empirical info is insufficient.
This is a Special Category amateur built helicopter and therefore you only have to prove so much in the design.