Well now you know why it was an open box deal. I hate that they do that crap but send it back while you are under the amazon window.
Im guessing there is no chassis intrusion jumper on the board, havent seen on of those for a very long time, like back in the days where you had to move a jumper to clear the CMOS and the chassis intrusion jumper was on the same header where you could connect it to a microswitch/proximity sensor if the cover is off.
There are a multitude of issues that can cause this other than it simply being a faulty board such as Fury suggested to remove any possibilities of other hardware shorting out, Fans chassis ground WC loop, etc. I would say that judging from what you have posted and it ran fine until you changed BIOS its one of two things.
One, you didnt go into bios and F5 for optomized defaults prior to updating the BIOS
Two, this is the reason it ended up listed as an open box because the original purchaser had problems and sent it back.
Some things Ill buy open box, mother board isnt one of them. New Egg had a special for a long time for the high end PG279Q monitors open box and had a butt load of them. Wasn't hard to read into that one. People were buying them, not being happy with bleed levels and swapping them out. Problem is for most places the open box warranty is not the same as a new one. The price difference just never seems to be worth the gamble to me.
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein