Why not try more than one distribution
Did you try with Molex power connector on motherboard connected?
Quote from page 3:
I was wondering, this white Molex power connector present on the motherboard, in the back close to the PCI express 1X, did you try to boot Linux with it connected?
There is also a 4-pin molex connector nearby in the event your power supply will just be providing a 20-pin and 4-pin power connector instead of a 24-pin and 8-pin interface.
It seems like in your running setup it wasn't:
On page 23 of the manual, it is sholwn connected.
Perhaps it is nothing ...