First, thanks for your support :)
It's still not working, I can't complete the installation (same error message as in my first post).
But it's quite normal I suppose.
What I want is Silverstripe to be installed in the my /ss folder for now, because I want to test if Silverstripe can be installed and setup without having to close or change the site for now. So even if it worked setting the rewrite base to / is not what I want.
Concerning moving the installation outside the root folder and / or change the httpd.conf, it's not possible. I'm using a shared hosting and can't do that :/
And I can't access anything using unrewritten URLs :(