Why is Rules Action "Create Linked Drupal User Account" not working?
The explanation for the issue you're facing is within the comment lines of the Rules Action you included iin your question, i.e.: 

  // @TODO What happens if they don't have an email at this point?

  // An email is a pre-requisite for a Drupal account, so the action
  // fails if they don't have an email.


The eMail ID for the CiviCRM contact you're using appears to be missing. So try again with a similar testcase, but make sure that for the CiviCRM contact some eMail Id is entered.

This probably also explains the "It was working some time ago": chances are that it was working, since you were using a CiviCRM contact for which an eMail ID was available.