I've got a puzzle. After updating to CiviCRM 4.7.27 (from 4.7.23) on WordPress, suddenly the civicrm cron jobs stopped being run. With some digging, I found that the credentials for the user I had set up to run cron were being rejected ("ERROR: Invalid username and/or password").
I confirmed that the username, password and site key were all correct (none of them had changed), but I still can't get it to work. I even created a new user with a new password, but that also failed. Both users have the correct permissions to access CiviCRM, etc. as specified in the documentation. I am even able to log in directly to WP using these credentials; they are only rejected in the context of a call to run cron.php.
I use the URL method of calling cron.php, which includes the username, password and site key in the full URL.
This isn't critical - I can still run it manually from inside CiviCRM, but I have already spent a lot of time on this and don't want to spend much more.
WordPress 4.8.3, with WordFence 6.3.21, all plugins up-to-date
CentOS 7, with the standard PHP 5.4.16 as supplied by the main CentOS repositories
Thanks in advance!