Thanks again, Charm and Sharon.

From Charm's description of her device, and from the fact that you both feel the pressure on your back teeth, the problem does seem to have to do with the way of using it.

I'm getting the picture now, it's all connected. Pressure at the wrong place leads to bending of the mouthpieces where it should not happen. That in turn leads to insufficient opening, and also to the tendency of teeth to slide outwards and cut through the pads.

She does remove her obturator while exercising.

No one demonstrated the use to us. I have managed to contact ATOS support here, and will ask them to give us a demonstration. Or fix/replace the device in case something's wrong.

Thanks again for the replies, it was really helpful!