Perfect LDLC service, but inadequate product.
It is important to understand the product description: "DisplayPort _to_ HDMI adapter". Even some technicians in the shop are fooled by this (who offered me a "DisplayPort _to_ HDMI cable" in exchange). This adapter (like the cable) only works in one direction: from a DisplayPort source TO the HDMI connector of a display. (and I needed the opposite...)
In the opposite direction (from an HDMI output to a DP input), you need an active, powered product.
The 10/10 is here for the LDLC service, which offered me a return against refund, without difficulty. The only constraint is to ensure the product is properly packed and taken to the post office.