850 no real milk foam

Hello
,we have the machine now for a few years. This milk foam problem I had once quite at the beginning. At that time I was advised to replace the outlet valve 2 bar with white marking. I did that at the time and it worked again.
Later, the machine has made fierce bitching. It constantly peed into the pomace cup. This was then with a new brewing unit better but not gone. Then the display was dark and I had to send it to Nivona. After that it ran fine for a long time. In the cup she pees when rinsing still now and then.
But now again the milk foam is watery and cold. I disassembled again the 2 bar valve with white mark and cleaned and decalcified it. It was better. I then ordered a new one and it was good at first. Now after about 3 weeks the milk foam is bad again.
Was the valve no good? Are there better ones, or I heard something about repair kits. They would be better than the new cheap valves. Is it due to this at all? What else could it be?
Am grateful for any tips.

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Nivona 850