Did you follow a specific article to set up your server?

You should check the error.log of your webserver (which shoudl be under /var/log/apache2/). This happens e.g. if your .htaccess uses unknown/wrong/forbidden variables.

If the script you're testing is in PHP, try setting
ini_set('log_errors','on');
ini_set('error_log','php-error.log');
at the beginning of your script (if it's really PHP) and reload the page. After that, check the php-error.log to see what's wrong.