Triggering a script over SSH on Ubuntu VM

I set up an Ubuntu Server 12.04 VM and installed the ubuntu-desktop package (the "Unity" graphical environment).
I also set up the Sikuli environment and wrote a Sikuli test that is supposed to open a RDP connection with some remote VM and interact with it : https://paste8.com/iIuCCMHi

- If I connect to the Ubuntu VM with my VNC client from my personal computer and launch the Sikuli test directly, it runs successfully.

- Now if I connect to the Ubuntu VM again but this time using SSH and launch the Sikuli test, it was first failing because it couldn't find the X server. So I wrote a bash script that launches the X server first so the Sikuli test can be started properly : https://paste8.com/Psjfcv5j

My intent is to schedule the execution of this script periodically using a cron job for example.

Here's the output though:

[log] openApp: rdesktop -u 'myuser@mydomain' -p 'mypassword' -g 1024x768 myremoteaddr
[log] App.open rdesktop -u 'myuser@mydomain' -p 'mypassword' -g 1024x768 myremoteaddr(27278)
[error] script stopped with error in line 2
[error] FindFailed ( can not find startbutton.png on the screen. )
[error] --- Traceback --- error source first
line: module ( function ) statement
58: Region ( wait ) Line 1574, in file Region.java
[error] --- Traceback --- end --------------

I guess Sikuli is not able to reach the screen content somehow. Do you know any workaround?