Ubuntu – Serial console configuration

So I have an Ubuntu server (12.10) running on Shuttle XS36V. This computer is totaly headless (just connected on the TV while installing).
Now, I'm trying to use the serial port to connect to my box (in case network is unavailable). By the way, that doesn't work at all 🙁

I followed https://help.ubuntu.com/community/SerialConsoleHowto

On the server :

root@server # dmesg | grep ttyS0
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-3.5.0-23-generic root=/dev/mapper/vg00-lvroot ro console=tty0 console=ttyS0,115200n8
[    0.000000] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-3.5.0-23-generic root=/dev/mapper/vg00-lvroot ro console=tty0 console=ttyS0,115200n8 
[    0.000000] console [ttyS0] enabled
[    1.720744] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    1.777564] 00:09: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A

I have also configured ttyS0 :

root@server # cat /etc/init/ttyS0.conf 
# ttyS0 - getty
#
# This service maintains a getty on ttyS0 from the point the system is
# started until it is shut down again.

start on stopped rc or RUNLEVEL=[2345]
stop on runlevel [!2345]

respawn
exec /sbin/getty -L 115200 ttyS0 vt102

On the client side (Ubuntu Desktop 12.10, but also tried Windows 7), I have just plugged the rs232 to usb cable and dmesg show that :

root@client # dmesg
...
[ 8722.807731] usb 1-1.2: new full-speed USB device number 5 using ehci_hcd
[ 8722.900863] usb 1-1.2: New USB device found, idVendor=067b, idProduct=2303
[ 8722.900872] usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 8722.900877] usb 1-1.2: Product: USB-Serial Controller D
[ 8722.900882] usb 1-1.2: Manufacturer: Prolific Technology Inc. 
[ 8722.945131] usbcore: registered new interface driver usbserial
[ 8722.945147] usbcore: registered new interface driver usbserial_generic 
[ 8722.945159] USB Serial support registered for generic
[ 8722.945163] usbserial: USB Serial Driver core
[ 8722.945517] usbcore: registered new interface driver pl2303
[ 8722.945532] USB Serial support registered for pl2303
[ 8722.945549] pl2303 1-1.2:1.0: pl2303 converter detected
[ 8722.947531] usb 1-1.2: pl2303 converter now attached to ttyUSB0

By the way, when running

screen /dev/ttyUSB0 115200 

nothing happened (of course my user belong to dialout group) … Only a blask screen.

Any idea ?
Thanks for help