Ok, when the issue occurs with the game even the benchmark don't work. I cannot disable Creator's Update because I don't have the windows.old folder anymore: when the issue appeared for the first time, I had the folder and when I've tried to roll back Windows was giving me an error at 30% of the process; after that, I had to clean install Win10 from a usb drive, and Mediatool made a version of Win10 with Creator's Update included (or it has installed it as soon as it boot up the first time, but in that case shouldn't it make the windows.old folder?).
Of course, Nvidia GeForce Experience and drivers are updated.
The PC was OCd via AiSuite, but after the clean install I didn't install it, neither PrecisionX for the EVGA 1080 FTW.
Regarding the BIOS boot at random, I've read that it can be caused by an usb device such as the keyboard: I've tried to unplug it before the boot, and it actually skipped the bios and loaded Win10 directly (except from the America Trend screen with the warning for the keyboard).

Still lost in this madness. Maybe if I upload the .evtx of FFXIV crashes someone can understand something from it?
Thanks for the support Xeon.