Does anyone know definitively, which lossless compression is used in the DNG converter?

The DNG specification states:

Otherwise, the JPEG variant must be lossless Huffman JPEG

However, there are TWO lossless compressions for JPEG (apart from JPEG 2000). There was the original method, defined around 1995, which was not very successful, but it worked; this was called LJPEG. Then, in 2004 a new method was adopted, called JPEG-LS or LOCO-I. Both methods use Huffmann encoding, but that's only part of the complete compression process.

I tried to ask this question from Adobe, but there is no proper "entrance" for that, because the DNG converter is not listed among the products.

The DNG spec (and Canon CR2 format, plus several Kodak raw formats) uses the old original lossless JPEG standard.

I suggest you download the DNG SDK and read the source code yourself if you want all the details.