EQ 300 Extra Class flashes and displays water tank

Hello everyone,

I have already used the search function but have not found anything helpful. My Siemens EQ 300 TI355F09DE/03 is suddenly causing problems.

After switching on, it rinses, but water only comes in through the drip tray, then it starts to flash and indicates Insert water tank.

Brewing unit (adjustment wheel is also at right angles, so I can't get the brewing unit out)

Water tank (float exposed), water tank inserted correctly, no additional water filter installed.
As soon as you reinsert the water tank, it starts rinsing again, but then stops again and flashes. I have done this several times in case there is air in the system. No success.

First measures: Removed and cleaned the silicone hoses. Water tank checked (lets water through into the machine), water level sensor is in the right place (can this be measured?) Voltmeter present.

Flowmeter present on this model? I have not seen this.

I hope someone can help me here.