Heat pump is not compatible with a water-bearing fireplace

Talking about efficiency and then constantly ventilating through the window. Exactly my sense of humor.

I have to admit we have always ventilated like that or opened the windows when it was too warm.

Today we at least ventilate somewhat properly and the consequence is double heating costs ;)

Yes, that's how my landlord in Singapore explained it to me back then: If it’s too cold for you because the air conditioning is blasting at maximum all the time, then just keep the sliding window slightly open.

Talking about efficiency and then constantly ventilating through the window. Exactly my sense of humor.

Should I rather let my food spoil? Sorry, but we don’t have a cellar. Feel free to tell me how I can do it better / correctly.

And how do you deactivate the ERR? Because in the storage rooms I don't need any (the is always open anyway) and normally not in the bedrooms either.

Deactivate ERR: Open the heating circuit distributor (usually a white flap visible from the outside, flush-mounted on or under the wall). You can easily unplug the actuators for the heating circuits that you no longer want to control via the ERR. Press the two tabs on the top sides with your thumb and forefinger; they usually pop off because the valve pushes them out. You can replace them with a handwheel/manual control cap for the heating circuit distributor and manually regulate the flow rate. The caps are available from plumbing suppliers for 3–5 euros.
You can unscrew the actuator cables at the switching panel and reinstall them if you need them again.

But actually, it doesn't matter as long as you have a window permanently open in one room (or several?). Then close the line for that room, but don't keep the window open permanently.

Should I just let my groceries spoil? Sorry, but we don’t have a basement. Feel free to tell me how I can do it better / properly.

Most groceries are not sensitive. For perishable items, only buy what you will use up quickly. And for the pantry, simply turn off the heating. Some items may need to be put in the fridge.