Intel S1200BTL server board with beep sequence 1-5-4-2

Hey there!
I built a server with an Intel S1200BTL server board, a 500W beQuiet power supply, an Intel E3-1220L and 16GB of DDR3 ECC RAM.

Now when I try and boot the server nothing happens for maybe 30 seconds (no fans spinning, just some LED flickering) and then the beep sequence 1-5-4-2 appears.

I already tried different power supplies but this doesn't change anything.

Anybody got an idea what the problem might be?

Thanks!