DisplayPort and its variant Mini DisplayPort, use a new and different protocol for communicating with your display.
A properly-functioning adapter can request the DisplayPort use "Dual-Mode", which produces a good approximation of single-Link DVI/HDMI. This is good for resolutions up to 1920 wide, which would include your Samsung (1600 by 900).
Wider displays require a more expensive "active" "powered" converter, to convert DisplayPort to Dual-Link DVI.
So if the display works with a straight DVI connection, but not a DisplayPort adaptation to "Dual Mode" DVI the weak link appears to be the adapter, but you cannot quite rule out the display card.
The connection says DVI-D on the monitor if that helps.
The DVI port on the computer offers both Digital (what most folks think of as "real" DVI) and Analog (VGA) signals on different pins on its connector. It is referred to as DVI-I (think "Inclusive" contains both).
DVI-D simply says that is ignores the redundant VGA signal.