I've programmed a calendar on my Stûv P-10, but it doesn't seem to work any more, even though the time slots are still there. Why does this happen?

I've programmed a calendar on my Stûv P-10, but it doesn't seem to work any more, even though the time slots are still there. Why does this happen? | pt

Once calendar programming has been activated, it remains operational except in the following cases:

You force the heater to operate outside the initially programmed time slots;
You stop the heater during an operating period;
The heater displays an error code.