Cron - Invalid key value sent
Site key should be defined in civicrm.settings.php, not settings.php, which isn't necessarily bootstrapped when cron is run.

If that was mis-stated in your question (likely, since you're using WordPress), you can try to troubleshoot this by debugging.


Find your CiviCRM log and your CiviCRM root.
In your CiviCRM root, find the file CRM/Utils/System.php.
Find this section (around line 553):

if ($key !== $siteKey) {
  return self::authenticateAbort(
    "ERROR: Invalid key value sent. " . $docAdd . "\n",
    $abort
  );   
}  

Add two debugging lines so that section now looks like:

if ($key !== $siteKey) {
  CRM_Core_Error::debug_var('key', $key);
  CRM_Core_Error::debug_var('siteKey', $siteKey);
  return self::authenticateAbort(
    "ERROR: Invalid key value sent. " . $docAdd . "\n",
    $abort
  );   
}  

Run your cron job again, get the same error.
Check the CiviCRM log.  At or very near the bottom of the log file, you should see lines that look like:

$key = 456;
$siteKey = 456;



...except I'm guessing that one of them looks slightly wrong!  Use that information to either fix this problem and/or update this question with more info.