Intergas Kompakt HRE Heater Problem: No Heating, Hot Water Works, Error Panel Display+Photos

Hello
I need help diagnosing a problem. I would like to advise you in advance that I am green in this matter and I would like to ask for quite blatant help.
Namely, the apartment has an Intergas Kompakt HRE heater, which has worked so far.
After returning from vacation, it suddenly does not heat, there is hot water, but the heaters do not heat. I found the instructions for it, but it tells me nothing at all.
I attach photos of the panel, which is displayed all the time. I suspect this is a mistake.
I tried to restart, but the stove literally turns on for a moment, it reaches the number 3 and turns off.
I would like a hint.

Here he adds a link to the instructions I found. Maybe someone will understand something more than I do.
https://www.fuego.pl/download/acv_kombikompakt_instrukcja.pdf

Hello
The boiler does not signal any error, the display shows the water pressure in the central heating system. The boiler heats the domestic water, because the boiler design enables it to operate and blocks the boiler operation due to the lack of pressure. First, let in water and vent the boiler and installation well (the boiler will start automatically when the minimum required pressure is reached), secondly, if a room thermostat is used, check the batteries and settings. This should help the boiler to start. If this does not help, call the service in the vicinity, although I see that the boiler is not installed in Poland.

Buddy, maybe you could suggest how to do it. Because this installation is black magic for me. There is a breather upstairs, I turned it on and nothing happened.
In general, the entire installation was replaced a year ago, so it is quite fresh. The room thermostat works.
After returning from vacation, the oven did not want to get caught, but eventually it caught up and he worked all day, the next day he did not start again.
I attach photos of how this installation looks like for me and this vent. Maybe you could say what to move and turn it off.
The installation is not exactly in Poland, which is why I would like to try to fix it myself.

Hello. The first photo shows the manual vent of the boiler. The second picture is more interesting - the valve with a black and blue knob is cold water, a little lower slightly to the right is the valve on the return which is enough to connect them with a hose from the washing machine, open both and complete the installation with water up to 1.5 bar (first switch the boiler in stand-by mode on the control panel by pressing the button on the left under the green LED, the display then shows the current pressure in co). After topping up, vent the boiler and radiators.

Thank you very much. I did as you said and everything works. I didn't think it could be fixed so easily .. Good lesson for the future.
Thank you again.