Happy ending!! After a week of trouble shooting, searching and several support calls, the solution came to me from a guy in a different forum.
So: Don't use the ATI provided DVI->HDMI adapter + HP provided HDMI cable. (I have also tried a different DVI-HDMI adapter and cables). Buy a DVI-HDMI CABLE and it will work !!! It has to be some wirings in the adapter that differs.