If you’re getting this message on your Apache installation and a 500 Internal Server Error message, you’ve used the ExpiresActive command in a .htaccess file or the apache2.conf (or httpd.conf) file without including the Expires module.
CacheRoot /var/cache/apache2/mc/mod_cache_disk CacheQuickHandler on CacheLock on CacheLockPath /tmp/mod_cache-lock CacheLockMaxAge 5 CacheIgnoreHeaders Set-Cookie CacheEnable disk / CacheHeader on CacheDefaultExpire 800 CacheMaxExpire 64000 CacheIgnoreNoLastMod On ExpiresActive on ExpiresDefault A300