What are you using as your IMAP & SMTP servers?
Try changing your Apache log level to 'debug' or 'trace8' for even more detailed error log.
But as everyone else said, this is definitely a permission issue involving your osTicket root directory in /var/www/ not being allowed access to PHP files..
Try this:
Before you do anything lets backup your www directory.
cd /var/www/
mkdir backup
cp -a /var/www/* /var/backup/
Now run this:
chmod 666 /var/www/site.de/osticket/upload/include/ost-config.php } change path to point to ost-config.php
chmod -R g+s /var/www/site.de
find /var/www/site.de -type d -exec chmod 0755 {} \;
find /var/www/site.de -type f -exec chmod 0644 {} \;