Jura C65 constant message: Open tap

Hello dear folks, our C65 has been giving the message "open tap", "system fills", "close tap", open tap"... This is how it goes on and on.
We have already removed the filter cartridge and also turned off the filter setting.
I have removed the flowmeter, cleaned it and reinserted it. I have thoroughly cleaned the hose from the water tank to the flowmeter, as well as the hose from the flowmeter to the pump and the one from the pump to the membrane regulator.
I just can't get any further and would appreciate any help.

Hello, Have you checked the contact points of the flowmeter on the circuit board and on the flowmeter? Otherwise, the flowmeter could be defective or the diaphragm pump is nearing its end.

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Unfortunately, I don't have this know-how when it comes to measuring.
The machine was only used very little. It is actually unlikely that the flowmeter or diaphragm pump are defective. The pump is working, because it pumps the water through the water outlet. And what could be defective in the flowmeter? There are only 2 plug contacts.

Hoses connected to the flowmeter the right way round again? Bottom of the tank in, top output to the pump.
Machine times decalcified? Machine can calcify despite water filter.

Yes, I had already run the descaling program 2x. And all hoses are connected correctly.
Yesterday and today you could get a cup of coffee again, then the same game started again.

Eventual or similar are often the reason for careless half-knowledge.
pump you can open. The nylon ball must be between 3.8-4mm diameter
O-ring is 6x2 mm. Flowmeter should run freely. Plug contacts must be free from corrosion.

Descaler should be amidosulfonic acid based. Lemon or lactic acid is something for the bathroom or toilet.
We are happy to help, but please then also process and then report on success here.

Thank you

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Unfortunately, I don't see how to open the pump. Unfortunately, there are no instructions either.
As a layman, I'd better not tinker with it. Afterwards it is leaky. There is a repair cafe near us next week, maybe I'll take it there.
Thank you for your time.