Also changing the Power management mode in NVIDIA controllpanel to "prefer maximum performance" seems to help. (even if it doesn't fix the issue completly, it does make the PC somewhat usable.)

Never Buying asus stuff again, that's what you can do. They had plenty of time since the RTX 4000 launch to fix this, they obviusly just don't care.

The PC Screen turns off, but Music keeps Playing for another couple of seconds, so it's not a PSU overload. After that the System reboots. Yes my BIOS is up to date.

Seriusly, thats not a solution!

I can confirm that my computer is essentially useless with a RTX 4090! Asus get on this, I don't want to change mobo to a different brand, just because of some compatibility BS!