Update: Nothing conclusive was found, and at this point, the diagnostic lights are purely complaining about some of the bit codes, which I was told was a non-issue by tech support.
Thanks to everyone for helping. My final conclusion is that the motherboard freaked out about having lost the battery for a few minutes, for whatever reason that is. I hadn’t unplugged the RTC battery, and I’d like to think those have enough juice to keep the motherboard active for more than 5 minutes…