External screen detected but not working on DisplayPort connection

I'm running up to date Raring on my Thinkpad X220 and stumbled upon weird problem.

I can connect to an external 24 inch DELL U2312HM external screen using DisplayPort cable.

I tried the same connection at work, with similar, but larger screen - 27 inch DELL U2713HM using the same cable.

When I check >>Settings >> Displays the display is detected. However I'm looking at dark screen.. Bizarrely - when I hit the PrtScr - I can also see it correctly with Ubuntu walpaper o_O

The monitor is detected and works correctly when using DisplayPort to DVI-D Adapter Cable (Single Link) however the resolution on this setup is limited to 1920 x 1200 so I'd really like to take advantage of DVI since the laptop should support it.

How could I fix this issue?