I have a XPS 15 9570 (2018) which is now out of warranty. All of a sudden it refused to boot flashing 2 amber lights and 4 white. So RAM problem.
I have only the 2 8gb factory ram sticks in it. I bought 2 new 16Gb Corsair ram sticks (Corsair DDR4 SODIMM Vengeance 2x16GB 2666)

I replaced the 2 8Gb with the 2 new 16Gb sticks => flashing 2 amber lights and 4 white
If I place one 8Gb stick in slot A or slot B => laptop boots fine.
If I place one 16Gb stick in slot A or slot B => laptop boots fine.
If I place two 8Gb stick in slot A and slot B => flashing 2 amber lights and 4 white.

If I place one 8Gb stick in slot A and one 16Gb in slot B => laptop boots fine.
If I place one 16Gb stick in slot A and one 8Gb in slot B => laptop boots fine.

So it seems that it only fails to boot when both sticks are in together of the same size. And so it seems to me that is not a ram stick issue.

What could this be? Can this be fixed ? Bios update?