Impossible to install piwik, error 500 (no replies)

I'm trying to install piwik on a new server, i already installed and used piwik with no issues so i don't understand.
This is exactly what i did :

cd /var/www
wget [piwik.org]
unzip latest.zip
rm How\ to\ install\ Piwik.html

Then i go to the url and i see "500 Internal Server Error".
I tried upload by ftp and chmod 777 and i have the same issue.

Here is my log file :

SoftException in Application.cpp:249: File "/var/www/piwik/index.php" is writeable by group
Premature end of script headers: index.php