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 :(
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. Your original .htaccess looks right for this.
Time to look elsewhere. You say in your original post:
I renamed every HTTPResponse instance to something else successfully, preventing conflit with the php config of my host. How did you do this?
Did you rename HTTPResponse.php to refelect your new name?
The following may just be a warning, sometimes a warning in error as mod_rewrite is active. If you get this when installing try accessing your root page directly "www.mysite.com" and chances are your install was successful regardless of the warning.
"Friendly URLs are not working. This is most likely because mod_rewrite isn't configured correctly on your site. Please check the following things in your Apache configuration; you may need to get your web host or server administrator to do this for you: "
* mod_rewrite is enabled
* AllowOverride All is set for your directory