Message "refill beans" NICR 660 Type 697

My Nivona NICR 660 Type 697 gives the message fill beans.
The coffee grinder is running and grinding the coffee.
The brewing unit rotates to the position to make the coffee.
The pump to make the coffee works for a very short time but then stops again.
The brewing process is aborted and the coffee powder is ejected into the waste bin.
A disc of ground coffee falls into the waste bin, certainly enough to make one coffee.
Is there a sensor or microswitch somewhere that allows the brewing unit to measure how much coffee powder is being compressed in the brewing unit.
With ground coffee through the valve on the side, the machine does work perfectly.
Thanks in advance for your answers.

If it works with ground coffee, then the grinder is probably grinding too little powder.

It is detected via a switch that is operated with the flag of the brewing unit, which looks out of the side at the top,

It is best to open the machine, dismantle the grinder, clean it and readjust it, or reassemble it using the markings.