Hello, ThuJoes.
Possibly, intel NUC DN2820FYKH on Windows7 or 8, When not connecting display,Graphic Driver can't work.
Similar,on Ubuntu12.04 Server and Desktop, When not connecting display,Graphic Driver can't work.
Framebuffer device seems to need working on Graphic Driver.
How about plugging the "HDMI Dummy plug" on HDMI connector?