This is the default error, it can mean practically anything - sort of "something is wrong".
You should see the details about the error in the error log.
If it is error in a script you could try changing the server settings and set the "display errors" option to ON, which would show you the line number and file name.
Alternatively, you could try it in PHP, but it does not always work, it depends if PHP can override the server setting.
ini_set('display_errors', '1');
error_reporting(E_ALL);