I get following two warnings:
Due to the settings for ServerTokens in httpd.conf, it is impossible to accurately determine the version of Apache running on this server. The reported value is Apache, to run Drupal without mod_rewrite, a minimum version of 2.2.16 is needed.
PHP OPcode caching can improve your site's performance considerably. It is highly recommended to have OPcache installed on your server.
It would be nice if someone could help me with some pointers about how to solve this problem.