EQ.6 plus S300

Hello, everybody,

I have bought an EQ.6 S300, which works as far as it goes. The only problem is foam formation. With Latte Macchiato the milk comes out pure and the machine makes whistling noises during production. To see / hear with this video:

https://vimeo.com/user107329582/review/383746314/1dfe7f3edd

The machine counts 11000 coffee productions and 27 milk products, so it was used exclusively for coffee. The brewing system is cleaned and the seals make a fit impression. Since one often reads something about the ceramic valve, I have already had this one apart. The ceramic plates were optically ok, common and no deposits were visible. But I'm just a layman.

Now I am at the end of my Latin and hope for help before I start to exchange parts.

TIA