ASROCK B450M-HDV R4.0 and Summit Ridge CPU

Hello everybody,

my freshly built PC shows no signal on the screen.
When I turn it on, the fans start spinning (normal speed, no full speed), but nothing else happens. The swapping of memory, power-supply and graphics card changed nothing. Also the replacement mainboard did not help.

The strange thing: Even without memory or without graphics card, the behavior is the same. Not even a beep from the buzzer.

I asked the same question in another forum. They suggested a problem with the UEFI version. The sticker on my mainboard says 3.20. And on their [URL=https://www.asrock.com/MB/AMD/B450M-HDV%20R4.0/index.asp#BIOS]website[/URL] Asrocks states: "ASRock do NOT recommend updating this BIOS if Pinnacle, Raven, Summit or Bristol Ridge CPU is being used on your system."

So I have two Questions:
1. Could an too new UEFI version cause the described behavior?
2. Is there a way to downgrade to an older UEFI version? (without a new CPU)

Configuration:
Mainboard: ASROCK B450M-HDV R4.0 (now the 2nd)
CPU: AMD Ryzen 3 1200 (boxed)
Graphics Card: PNY GeForce GTX 970 4096 MB GDDR5 (alternative: Gigabyte GeForce GTX 1060 OC WindForce 2X, 6144 MB GDDR5)
Memory: 1xKINGSTON Server Premier - KSM29RS8/8HCI - 8GB 2933MHZ (alternative: 2x Micron 8GB 1Rx4 PC4-2133P-R / PC4-17000R DDR4 Registered Server-RAM Module REG ECC - MTA18ASF1G72PZ-2G1A2KJ)
Power-supply: Be quiet! Pure Power 11 400 W CM (alternative: be quiet! 350 Watt Pure Power L8 Non-Modular 80+ Bronze)

CliffClavin