Asus Chromebox 3 not posting after installing UEFI Firmware

I have an Asus Chromebox 3 CN65 (the one from a Google Hangout kit, don’t know if this is relevant) on which I want to install an UEFI firmware to use a different OS. I used the Firmware Utility Script from MrChromebox’s website. It apparently installed succesfully but after a reboot it didn’t POST for >hour. I restored the backup using a CH341A programmer and tried one more time but it didn’t work again. Then I tried to flash the UEFI firmware using the unbricking guide but I didn’t succeed yet again. Everytime I leave it for a long time, nothing happens. When I hold the power button to shut it down the fans ramp up to 100%, stopping when I let go off it.

What could I be doing wrong?