VNC Desktop on AWS

Question

I've installed ubuntu-desktop and tightvncserver on my Amazon AWS instance, I've configured Putty to forward port5901 and that all seems to work fine, I can connect but instead of seeing the Ubuntu desktop, I only see a solid grey screen with the X cursor. I have set my ~/.vnc/xstartup to execute gnome-session --session=ubuntu-2d but for some reason this doesn't work, why not I'm wondering...

EDIT 1

It is an instance of Ubuntu 14.04:
$ uname -a Linux ip-10-0-0-145 3.13.0-36-generic #63-Ubuntu SMP Wed Sep 3 21:30:07 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

I double-checked the following:

 $gnome-session --session=ubuntu-2d

** (gnome-session:11133): WARNING **: Cannot open display:

and

$ ls -l /usr/share/xsessions/
total 24
-rw-r--r-- 1 root root  216 Mar 26 17:37 gnome.desktop
-rw-r--r-- 1 root root  272 Sep 30  2014 gnome-fallback-compiz.desktop
-rw-r--r-- 1 root root  310 Sep 30  2014 gnome-fallback.desktop
-rw-r--r-- 1 root root  213 Mar 26 17:37 ubuntu.desktop
-rw-r--r-- 1 root root 4294 Mar 21  2014 xfce.desktop

the complete xstartup now looks like:

$ cat ~/.vnc/xstartup
#!/bin/sh

xrdb $HOME/.Xresources
xsetroot -solid grey
#x-terminal-emulator -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
#x-window-manager &
# Fix to make GNOME work
export XKL_XMODMAP_DISABLE=1

EDIT 2

Changed xstartup to look like:

$ cat ~/.vnc/xstartup
#!/bin/sh

xrdb $HOME/.Xresources
xsetroot -solid grey
#x-terminal-emulator -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
#x-window-manager &
# Fix to make GNOME work
export XKL_XMODMAP_DISABLE=1
gnome-session --session=gnome-fallback

killed and restarted the server with $ vncserver -kill :1 and $ vncserver :1 but am still getting the same solid background only.

Instead of using gnome-session --session=gnome-fallback, I tried xfce4-ssssssssseion and that work just fine. I.e. it's some problem with gnome... I also tried to start it manually but got:

$ gnome-session --display=:1

(gnome-session-check-accelerated:12077): Gtk-WARNING **: cannot open display:

(gnome-session-check-accelerated:12081): Gtk-WARNING **: cannot open display:
gnome-session[12065]: WARNING: software acceleration check failed: Child process exited with code 1
gnome-session[12065]: CRITICAL: We failed, but the fail whale is dead. Sorry....