Before, you could turn on the system by pressing the power button once. Always.

Now when I press the power button, it turns on briefly, fans start to power up etc, but it turns back off after less than one second. Pressing it again causes the system to power up normally.

Curiously, I see the same behavior for a different computer I am building - same processor / motherboard but with a different case and with a Zalman 600W power supply.