Vostro 3470 SFF "Memory failure" error after adding second memory stick

I'm getting a Vostro 3470 SFF "Memory failure" error after adding second, identical to the first one, 8GB memory stick.

Either memory module is recognized in DIMM1 slot by themself, but when the second one is added, I got a "memory failure" error  (2, 7 blinking amber light).

The memory the PC had when I got it is an SK-Hynix 8GB 2666MHz HMA81GU6JJR8N-VK, so I got a second, identical one.

Both stick is fine (seems like), and having both of them installed is a supported configuration based on what I read.

My PC has a Core i5-9400 CPU

Any suggestion is greatly appreciated before I'm buying a set of different (Crucial) memory