What is wrong with my heater?

I've seen all that before... And getting that part back in while the water fountain is coming out there is not so easy.

as you yourself correctly quoted "push in" and not pull out with force. Please read first, then write. And the people here are not as immature as you think.

I also think the return and supply are reversed, I have a video of it somewhere, but for us it was the heating coil on the [Kontrollierte-Wohnraumlüftung].

Whoever pushes something in with pliers can just as quickly pull it out... Small cause, big effect! I've actually seen live how a few liters of heating water emptied into an apartment from a fountain because someone pulled out this pin. And that's what I wanted to warn against. Such things happen faster than you think if you've never done it before. And then you’re not the one responsible for the damage

All good, on Monday the professional called by the landlord will come and repair it. Thanks to all of you for the quick and helpful tips – really nice!