Your boiler has shut down because water isn't moving through the system quickly enough, causing it to overheat too fast.

Technical description

Lock-out: appears if the maximum temperature rise is exceeded or if the dT between the supply and return temperature ≥ 45°C or a rate of rise in the flow temperature > 1°C/sec. And also at a boiler-start with no through flow or insufficient water. The boiler will try to restart after 10 minutes.