Oh... How stupid am I? Initially I thought it was too obvious to be the solution to my problem! I have wired everything up (except for the cube, which currently not connected), compiled the code and disconnected the PCB from the USBasp. Now, I am using a 5V 3A power supply and the only visual result is the two LEDs on the switch flashing alternately and infinitely. When I press the reset button it starts all over again (as expected), but as long as I press the button everything keeps being switched off (apart from the power LEDs). Also, when I press the switch with the 2 LEDs, it stops flashing and only 1 LED is kept on. According to the code (which I am not yet capable of undestanding) is this the way the circuit should behave? Keep in mind that you don't have to answer if it's too difficult find out whether there's a problem with the circuit or not. I will soon connect the LED cube and I'll let you know if I encounter any kind of issues. Thank you VERY much for your help!