Livemixer B650, Ryzen 9600X no boot/post

Hello everyone,
I am now quite desperate and hope for finding support via the knowledge here in the forum.
I assembled a new PC at the beginning of January 2025:

New parts:
??AMD Ryzen 5 9600X
??ASROCK LiveMixer B650 (installed Bios Version 3.16)
??2x 16GB Crucial CT16G56C46U5 DDR5-5600 DIMM
??Arctic Freezer 36
??550 Watt be quiet! Pure Power 12
??Lexar NM790 2TB (system) and 4TB (data)
??Be quiet Pure Base 500DX
Used:
??Gigabyte Geforce RTX 3060 12GB

The computer ran perfectly until about 3 weeks ago. I used it every day for work, and I was finally in the process of playing through Shadow of Tombraider completely.

I can remember a single issue, that was at the beginning of March. There, after shutting down Windows 11 (24H2), the power supply to the USB ports where still powered on and the fans were also running.
PowerSaving is activated in BIOS, USB power is switched off. Just like fastboot, or fast start and hibernation in Windows. The ram ran without overclocking because of the DDDR5-5600, so I didn't activate an EXPO profile.

In April, the problems have been getting worse. No changes at the hardware and/or BIOS. I did not even move the PC. The issues manifest themselves as follows:

??Turn on the computer
??Fans turn on, USB ports are supplied with power
??LEDs of the motherboard go through the self-test (CPU, DRAM, VGA seems ok) and then get stuck at boot (LED boot stays on red). There is no post (or biosscreen I would expect if no data carrier is found). Both monitors remain black.
??The shutdown from Windows does not always work either, i.e. system shuts down but fans and USB ports remain on
??Usually a power-off on the power supply helped. The next start usually worked.

Since the beginning of last week it now seems to be a complete failure. Both screens stay black, the system hangs on boot (LED boot stays on red) and no longer starts. I tried the start the PC for more than 20 times.
Despite the startup problems, the PC worked without any problems after a successful startup. The Windows Event Viewer did not show any errors. The reliability curve also looked good. Only once did I find an error about the TPM2 module.

What have I tried so far:

??Removed the Gigabyte RTX and used internal graphics of the Ryzen 9600. The Ryzen GPU has been working fine, at leaste fort he inital installation. I added the RTX after Windows has been up and running.
??I removed both SSDs ??both run perfectly in an M2 USB case on another PC
??Reduced the RAM to one DIMM ??I also swapped both: so I tried both individually in slot B2

For me, the question is what to do next? I think either the board or the CPU has a problem. How can I narrow this down further?

Thanks for any suggestions.
Thank you! And sorry for my bad English in advance.

what debug light/s exactly light up and in what order? what does manual say about it?

elendiir wrote: A short update: I just used the bios flashback option to update the BIOS of the board from version 3.16 to 3.25. For me the flashback has been successfully. After pressing the flashback button the flashback LED started blinking slowly (reading the BIOS file?) and then I did see a faster blinking (programming the BIOS?). After roughly 5 minutes the LED was off. I did reset the CMOS but still the same: black screen, no post, red boot status LED lites up ad does not turn off.

idk, read the manual

https://www.asrock.com/support/QA/FlashbackSOP.pdf