Yes, hard to say what the reason for the black screen, HDMI-VGA-DAC or the LCD's „smart“-VGA-electronic. Okey, i could lend a DSO..., but not sure if it is worth the effort... Finally, with force_scandoubler option, the image is quite good.
The monitor has no hardware caused scanlines by the way.
Shame on me, i messed up the numbers. I am sorry! The Monitor is a Syncmaster 943N. It has only a VGA-Port.
But if direct_video uses resolutions for 15 kHz to go over HDMI specs where as the VGA-board generates the analog video signal in a true 15 kHz hsync way, it gives me a reason for a black screen over HDMI-VGA-DAC at least.