Piwik couldn't write to some directories.
Try to Execute the following commands on your server:
chown -R www-data:www-data /data/www/sigal/piwik
chmod -R 0755 /data/www/sigal/piwik/tmp/templates_c
chmod -R 0755 /data/www/sigal/piwik/tmp/cache
chmod -R 0755 /data/www/sigal/piwik/tmp/assets
chmod -R 0755 /data/www/sigal/piwik/tmp/tcpdf
If this doesn't work, you can try to create the directories with your FTP software, and set the CHMOD to 0777 (with your FTP software, right click on the directories, permissions).
After applying the modifications, you can refresh the page.
If you need more help, try Piwik.org.