it might be software issue ?
like card bios issue ?
no, I don't think because of this
i have this kind of artifcacts using risers,with 1 gpu it boots up, when i tried 2 or more this kind of bug appears and windows freezes ( tried ethos same error)
try plugin only one gpu to motherboard direct, without risers
It worked for me
And looking your riser, i think you should plug only one 12v rail, the 4 pin one, not the 6 pin