Ubuntu: xorg - cannot install controller configuration CRT 262

Question

I have a server running under ubuntu with no hardware display connected. Basic display resolution in this case is

$ xrandr -q
Screen 0: minimum 640 x 480, current 800 x 600, maximum 800 x 600
default connected 800x600+0+0 0mm x 0mm
   800x600        61.0* 
   640x480        60.0  

But first is a bit low for me - i want to set 1024x768 to use teamviewer and vnc comfortable. So i have tried to add new resolution:

$ cvt 1024 768 60
# 1024x768 59.92 Hz (CVT 0.79M3) hsync: 47.82 kHz; pclk: 63.50 MHz
Modeline "1024x768_60.00"   63.50  1024 1072 1176 1328  768 771 775 798 -hsync +vsync

than check and add mode:

$ xrandr -q
Screen 0: minimum 640 x 480, current 800 x 600, maximum 800 x 600
default connected 800x600+0+0 0mm x 0mm
   800x600        61.0* 
   640x480        60.0  
  1024x768_60.00 (0x120)   63.0MHz
        h: width  1024 start 1072 end 1176 total 1328 skew    0 clock   47.4KHz
        v: height  768 start  771 end  775 total  798           clock   59.4Hz

xrandr --addmode default 1024x768_60.00
uadm@spravka:~$ xrandr -q
Screen 0: minimum 640 x 480, current 800 x 600, maximum 800 x 600
default connected 800x600+0+0 0mm x 0mm
   800x600        61.0* 
   640x480        60.0  
   1024x768_60.00   59.4  

so now i can pick 1024x768 in my Display configuration gui.. but it don't works. Someone can tell me where is a problem?

PS: when i connect hardware display - it sets normal real resolution such as 1280x960 automatically. Thanks for help.