fatal errors in installing ubuntu server on virtual box

I had no issues when I installed ubuntu desktop.
But, the fatal error I got installing server version is too frustrating...
I copied the bottom part of vbox.log.
Hope it would help to find the solution...


00:00:21.433953 VGA Graphics Controller (3CF): GR index 3CE:05
00:00:21.433953 GR00:00 GR01:00 GR02:00 GR03:00 GR04:00 GR05:50 GR06:05 GR07:0F GR08:FF
00:00:21.433956 !!
00:00:21.433956 !! {vgapl}
00:00:21.433957 !!
00:00:21.433957 read mode : 0 write mode: 0
00:00:21.433958 set/reset data: 00 S/R enable: 00
00:00:21.433958 color compare : 00 read map : 0
00:00:21.433959 rotate : 0 function : 0
00:00:21.433960 don't care : 0F bit mask : FF
00:00:21.433960 seq plane mask: 0F chain-4 : on
00:00:21.433961 !!
00:00:21.433961 !! {vgasr}
00:00:21.433962 !!
00:00:21.433962 VGA Sequencer (3C5): SR index 3C4:04
00:00:21.433963 SR00:03 SR01:00 SR02:0F SR03:00 SR04:0A
00:00:21.433964 !!
00:00:21.433965 !! {vgatext}
00:00:21.433965 !!
00:00:21.433965 Not in text mode!
00:00:21.433966 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!
00:00:21.433981 emR3Debug: rc=VINF_EM_TRIPLE_FAULT
00:00:22.433827 Changing the VM state from 'RUNNING' to 'GURU_MEDITATION'.