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

If I interpret the drawing correctly, this exact ring is attached from below, then the chipboard, and everything is tightened with the nut.

Unfortunately, you are interpreting it incorrectly. In the drawing you attached, it is clearly visible that the centering ring is supposed to go into the base of the faucet (where it is already factory-installed and does not need to be mounted by me). The base sits ON the washbasin and is centered in the washbasin hole by this centering ring and sealed against penetrating water using the gasket. What is the gasket supposed to do under the basin? o_O Not to mention that it would not provide any support, block the way for the flexible hoses, and break when tightening the chipboard.

No no, the installation sequence is very clear and straightforward:

Faucet with centering ring in the base > from above onto the washbasin < fastened using chipboard and nut from below

Hmm....then I don't know what else to do about what is going wrong. Honestly, I've never come across that before. There will be no other choice but to tighten the [Tei] firmly or to commission a professional with it. However, I would still ask you to share the cause if found.

or just hire a professional for it

Which brings me back to an .

In my opinion, there are only the 2 options.

1. it simply hasn't been tightened enough so far and that's why the faucet hasn't been straightened yet
2. contrary to your previous opinion, the plastic ring must go under the sink while the gasket sits on top.

It shouldn't be a problem to try that out.

2. contrary to your previous opinion, the plastic ring must go under the basin, while the seal sits on top.

I for God's sake don't want any quarrel and am happy to be taught, but how is that supposed to work? Plastic ring and seal are one part. And how do you come to that conclusion? The drawing clearly does not show that!?