C15KS61N0/04

Hello, I'm new here in the forum. We have been using the above-mentioned Neff coffee machine privately for eight years now, the machine has just under 11000 cups behind it.
Regular cleaning and descaling have been carried out. Recently there has been a problem, you want to make a coffee, the machine grinds, a small amount of coffee comes out of the spout, then despite the full water tank the message Refill water without carbon dioxide or replace filter appears, the door is then opened and you remove the water tank and put it back in, the machine goes to abort and the normal display background appears, after which you can then take a coffee again, the Auotmat then does what it should.
Perhaps someone from the forum has a tip as to where the fault could lie .
Mfg
Ralph