Long story short, new build, PC started, installed OS, updated OS, some low loads for less than an hour (installing programs, browsing the internet, listening music videos meanwhile), than a very normal shutdown.
After about one hour, just tried to power it up again, it didn't and it didn't anymore.
Paperclip and multimeter, the RM550x spun, then stopped, DMM readings are 3,2V / 12,12V / 5,05V / -12,21V, while connecting test leads to PWR_OK gives 4.97V
I guess it's all correct, right? Might it be a PWR_OK timings issue? If so, might I investigate it with just the DMM? Mobo is an ASUS Z170i Pro Gaming.