Then Gigabyte told you to post here? Dunno, really sounds like a hardware error related to the board. Switch, jumper, setting. You sure the memory sticks are identical? Does the memory have to be installed in any order?
If everything runs fine with 16 GB in, maybe you should try uninstalling the toolbox. You've run the system file checker and check disk, with nothing unusual found with 16 GB, right?
Set BIOS to defaults, save that, reboot. You've done that, right, and then made whatever tweaks you run? Really sounds like a setting. I dunno, just shoving ideas at you.