We have a couple of (identical) Ubuntu 11.10 machines, configured with gnome-classic, which we use as remote servers, and let our clients log into personal user accounts we create for them using vnc4server.

We configured all the machines in the same way, following a short manual we compiled, describing how to download, install and prepare a few tools and our software.

The connection usually works fine, but today I set up a fresh machine, and experienced problems.

After installing vnc4server, I ran vncpasswd and copied the following startup file to ~/.vnc/xstartup:

#!/bin/sh  
unset SESSION_MANAGER  
unset DBUS_SESSION_BUS_ADDRESS  
gnome-session --session=gnome-classic &  
[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup  
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources  
xsetroot -solid grey  
vncconfig -iconic &  

Then, I started vnc4server and used two viewers (the Ubuntu Remote Desktop Viewer and Windows RealVNC Client) in two other machines, but instead of getting my desktop, I see an empty window with a grey-ish background pattern like this:

and the cursor is a bold X.

What is wrong with the setup and why don't I get a remote session as expected?