Titan109 is right. For me, the card does not run on the ASUS MAXIMUS HERO X mainboard. Unfortunately, the compatibility lists in the printed version and on the Internet are not identical. It is essential to clarify exactly whether the card runs with your board. But that will be difficult if there are errors in the documentation. In general, this is the worst product I have ever bought from ASUS. But otherwise I was always very satisfied with ASUS.

The connection of the card works via PCIe 3.0 x4 and thus actually in each of the slots, but in addition a control via system link cable via Thunderbolt port of the mainboard is necessary. This is the reason why the card only works in certain boards - and only from Asus.

Hello everyone and thank you for your answers "a bit late" in hindsight!

It is besch.... The compatibility lists are wrong and outdated in different ways everywhere. On my board (X99-E WS/USB 3.1) it says "Excellent memory flexibility - Integrated M.2 x4 standard and support for ASUS PIKE II and ThunderboltEX II cards". With the ThunderboltEX II (/DUAL) cards, however, there is not even a single X99 board, with the latter even only one Z87 and two Z97!
It's quite different with the ThunderboltEX 3 (on the ASUS page, less so elsewhere), there the list is "relatively long", including my model with * (and incidentally also MAXIMUS IX HERO, MAXIMUS VIII EXTREME, MAXIMUS VIII HERO, MAXIMUS VIII RANGER).

Have you tried the card in different slots? Have you looked in the manual? Or do you not have the Thunderbolt header?
Do you still have the card lying around? Then I could maybe test it on my board...??