U2412M, DP not working, enters PSM

Hi,

I have two U2412M REV A01 working good with VGA and DVI cables. But, when I connect the DP cable, the U2412M tell me PSM (power save mode). Both U2412M tell me this text. I have used more and more DP cable. So how I can resolve this problem? Can you help me? Do I need a firmware update or what?

Thank you in advance

Scilla