It's the power supply. I had the exact same problem with 3 cubes, many others here in this thread same, problem, usually with a Seasonic and also with some of the Corsair's with an "X' in the model name. Usually a rebadged Seasonic.  It would run for 1.75 seconds then die once turned on or once the fuze was inserted.

Don't waste your time openeing up(well you can check that everything is tight and the cards are in their slot) , it is not the reason for the problem you are having now. If you go back in the thread you can see there is a solution by putting some Capacitors in line with the power at the back of the cube, but it is a pain in the ass and can be fiddly.  

The two power supply's i have tested and are good to power 2 cubes at a time are the Corsair GS 800  and the thermaltake 840. Both worked fine for me. My Seasonic 1050w was a royal pain in the ass.