I'm answering to myself (for people with the same problems):
- First of all, at the install step, it was always blocked at "Building database schema...". No errors, no info on forums, etc...
After manual researches (thanks PHP logs!), I've actually found that it was searching for something called "Auth/OpenID.php"... No information in the requirements about this, but I downloaded it here: http://openidenabled.com/files/php-openid/packages/php-openid-2.1.1.zip and the process went ahead!
- Second problem (this one), the installation seemed OK, but still a blank page on screen...
PHP logs again + debugging... and I understood that there was a conflict with the HTTP extension in our PHP installation (pecl_http).
In my case, I've just commented the extension in /etc/php5/conf.d/http.ini and then restarted Apache...
Everything seems to work now!
Hope it will help someone!