Garage Years:
First question - how would anyone prove the ash concentration that caused such an incident was above or below whatever (arbitrary) level was stated to be safe. Not being flippant but the evidence has a tendency to blow away.... i.e. the ash cloud itself.
Very, very simple. Erosion and overheat damage to turbine blades and nozzle guide vanes associated with glass clogged cooling holes.
By the way, the diffusion of responsibility between engine/airframe/ Met office/airline/insurer/crew and regulator in the event of a series of extremely costly Ash damage incidents is exactly what has kept you grounded until enough research and negotiation could be done to understand and manage the risks.