Mini PC router troubleshooting

[Expert0]

I'm using a Celeron J1800 mini PC as a router for WAN failover. It has been stable until today where it suddenly crashed. I tried an HDMI connection to see what was going on and got a "XZ compressed data is corrupt" when it tried to boot into the OS (Debian based). A few reboots later, I can't even get to post. I have tried replacing the memory module but doesn't help. The mini PC does power up and I connected a fan and got it to spin too. I tried removing the RAM and SSD but I can still get it miniPC to power up the same way. The motherboard seems generic and I can't find a brand name on it anywhere.

 

There used to be a beep at startup which I no longer get. There are a few LEDs on the motherboard and one on the SSD that light up. The LAN ports also appear to work fine. The USB ports on the other hand, provide power only for a second when I plug in the a and then there is no power. 

 

What could be wrong? 

1) The obvious guess would have been RAM but I tried a replacement which gets me nowhere.

2) SSD? But I suppose a faulty SSD shouldn't affect POST.

3) CPU?