Melitta Caffeo uneven coffee output

My appreciated Melitta Caffeo Varianza CSP has developed the quirk, that the the cups are not the same size. It's typically the first cup that is both smaller and weaker in taste than the second cup. I did change the drain valve and cleaned the area around it, as it was a lot of excess coffee there, but this did not help on the original problem.

Could someone help me pinpoint the origin of the problem ?????

photo of the problem

Hello.
Uneven coffee dispensing may be due to the following problem.
Flowmeter dirty, partially defective or plug contacts corroded. Remedy: Clean.
The cycles have not been run with the correct descaler. Use Calc'n Clean with
descaler with amidosulfonic acid and cleaning tablets for the brewing unit. Remedy: Run Calc'n Clean
The pump capacity must also be taken into account. Which pump?
Grind quantity? Remedy: Brewing unit must run smoothly. Possibly brew a cup 10 times without beans in the grinder. With the lowest bean level.
Pomace puks? =cm?

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Always ran all cleaning and descaling programs.
I also just cleaned the grinder and pathway to the brewing unit.

It could be the pump or something with setting the pressure, I hear a sound like it's release some pressure just before it pumps water through the coffee.