Issue with air entering the coffee machine system EQ6 Plus s100

The machine says "Refill with still water or change filter".
I don’t have a water filter installed in the machine, and I’ve filled the water tank with regular still water.
My issue is that air is entering the system. I can hear a bubbling sound inside the machine, and some air is released during coffee brewing and when dispensing hot water.
I’ve performed a test where I disconnected the hose between the pump and the heater. I placed the hose in a glass of water, and small bubbles appear in the water.
I’ve observed the water hose from the tank and cannot see any air entering the water from the tank.
So the air must be entering the system somewhere between the flow meter and the heater.
Where could the air be entering the system? Could it be that the pump is faulty?