If this is a fresh server, make sure that you have set correct locales and timezone.
You can check the installed version number of proxmox by logging into the proxmox control panel provided by online.net, at https://yourboxid.dedibox.fr:8006
If the version number is 2.3-13, just follow the following instructions:
Upgrade 2.3 to 3.0:
Get the upgrade script:
Now, add permissions and run it:
chmod +x pve-upgrade-2.3-to-3.0 ./pve-upgrade-2.3-to-3.0 --download-only ./pve-upgrade-2.3-to-3.0 reboot
Note that your Debian squeeze gets automagically upgraded to wheezy.