Sealing problem with washbasin single-lever single-hole mixer 01092283

I can't imagine that

But it is. The pictures show it clearly.

The plate (part 5.3 - the white plastic piece with the gasket) is factory-installed exactly where it sits - on the mounting threaded pin in the base of the faucet. My assembly order from top to bottom:
Faucet > Washbasin < Crescent washer < Locking nut.

Edit: For all readers wondering what is meant by "in description 5.3," here is another assembly diagram that I unfortunately cannot add to the first because the editing period has expired.

Geez... one should be able to figure out what the problem is, it annoys me now... I have to check at home if I can find something in my documents, but it could also be tricky, man.. :)

or someone else chimes in who has the insight. Nothing else comes to mind spontaneously right now...

Thank you, that is really very kind of you!

Maybe the white board does have to go under the washbasin after all? At least with our (Keine Hansa-Armatur) there was also a plastic part between the washbasin and the crescent.

That is hardly imaginable because this component is the seal with which the fitting AUF is supposed to be sealed against penetrating water at the washbasin. Under the table, the counterpart is only the crescent. If you followed your thought and removed the part, there would be no sealing at all, which is complete nonsense. Furthermore, there is no additional ring seal included that could be considered as an alternative.

With this small number of fastening parts (2 in total), you really cannot do anything wrong because the fitting is completely pre-assembled and only the crescent and the nut are included as fastening material.

The seal naturally still has to go on top. I also had it the wrong way around with our faucet at first, since it was delivered assembled that way to us. I was then surprised when tightening the screw that the metal washer was scraping against the ceramic like that. After looking at the instructions, it was clear to me that the plastic still had to go in between the metal half-moon and the ceramic.