So I've talked with the RMA and they assured me that the BIOS was upgraded twice and the latest version is 4.5

I have acquired a speaker from work, and borrowed a dedicated GPU.

In order to not introduce to many variables at once, I started out by attaching the speaker.
Tried a new HDMI cable. And I reversed the button button connector... The latter was thought to have no impact since it just leads to ground.

I pressed the start button and the speaker gave me 5 beeps , I could see a message on the screen telling me to insert boot device. HURRAY!

The 5 beeps was because I hadn't connected a keyboard yet. (in earlier attempts this was connected though)

The odd thing is that during the weekend I tried to boot/flash with an A6 , but this just gave no POST and the system kept running (same problem as you). This is what lead to the retailer flashing the BIOS for me.

I Should inform that I am powering it with a picoPSU and the new HDPLEX H1.Sv2 case. It might be that the case power-on switch has a rectifier only allowing the current in one direction 

Then again how could it power on in the first place...

I thought about clearing CMOS after I got it back from RMA, but the manual advised against clearing a newly flashed BIOS. So I waited till I got the speaker and GPU.

Now the system is running and I am currently monitoring the CPU temperature to see my idle temp. with a fanless system :)

Thanks for the help guys...

Also, this appears to be a new symptom! Mine simply kept going and the only way I could shut it off was by switching off the power unit.

Keep us posted, I would be really interested in finding out what the problem is.

I'm sorry I did not post anything else, but I did not really find a solution.. I ended up sending back the motherboard and I simply ordered an Asus Prime A320M-k (not a miniITX however) and everything worked perfectly. Apparently the Asus already came out of the box with Raven Ridge support, although I did not test it since the a6 9500 was the first processor I installed.

Anyway, the new board recognized everything (processor, RAM, integrated graphic unit) immediately and everything and is working fine. I only get occasional crashes while playing some steam games (oblivion, Skyrim, Civ 6 so far) but I presume it has something to do with some driver since windows does not freeze..

Everything is now working fine so I can only assume that the problem was the motherboard, wether it was a flawed Bios or a damaged board I cannot tell. 

I hope ASRock fixes this problem quickly.

I'm sorry I cannot really help you!

In my case:
The shop has upgraded my BIOS to the newest version, although I didn't see proof, nor could they tell me which version it was upgraded to.
I will call the guy who did it tomorrow and ask directly, supposedly it should be ready for Ryzen 2200G since this was the agreement with the tech guy. Even told him it should be flashed twice in order to be the latest.

I tried with one RAM, with, 2, with the 4pin PU, with the 8pin PU, with the extra graphic card, with 2 different monitors.. nada. I guess my mainboard is just broken? but then again, how did the shop manage to make it work!?

I tried to call the tech support but it wasn't really helpful, they said I should just send it in once again since they don't know who was working on it...

I'm really thinking about returning everything and try again with new components.