Coolant overflowing through the reservoir?

I have an: 318d - 122hp/90kw -m47engine- BMW - 2007. The coolant disappears from the reservoir after a 20km trip, yellow warning on dash. No leak, no smoke, no oil level increase, no oil in the coolant. I let the car run without the coolant cap, topped at max - and it stayed like that, then car heat up to 95*C degrees, coolant came up the reservoir bursting out, overflowing. Do you have any ideas?