Take the following contents, put them into a file named something.php in your store's root directory; then run www.yourstore.com/something.php.

It will let you know if the error-log file name was properly set.
Code:
<?php
require 'includes/application_top.php';
$current_error_log = ini_get('error_log');

echo "The current error-log file is $current_error_log.<br />";

if ($current_error_log != $debug_logfile_path) {
  echo "The current error log is not set correctly, it should be $debug_logfile_path; retrying ....<br />";
  ini_set('error_log', $debug_logfile_path);
  echo "Reset error_log to " . ini_get('error_log') . '<br />';
}

require 'includes/application_bottom.php';