Long story short-ish,
Upgraded from 3080 to 4090, changed nothing else. Booted up, got into Windows, then black screen. Tried that a few times, booted into Safe Mode, uninstalled driver, back into regular mode, installed driver, black screen or froze… did that like 5 times.
Then formatted c drive, installed Windows, updates, drivers, then Nvidia driver, black screen… same story.
Tried my friends pci-e extender, same again (both Asus 4.0 16x cables).
Now the card is directly in the pci slot, and seems to work fine, did several 3DMark runs, including OC, all passed.
I really want the card mounted vertically, so anyone who knows a fix?