I am using Drupal 7.56 and CiviCRM 4.7.20. When I execute the mailing scheduler it appears to complete successfully but no email is sent. It fails with this error:
line 81 of /var/www/html/sites/all/modules/civicrm/CRM/Mailing/Info.php
I have had a look at the code but have no coding skills to understand why this is happening. The circumstances are odd. I upgraded to 4.7.21 and it appeared to succeed however when I clicked System Status - Warning it displayed nothing. I then tried to continue a draft Mosaico email and it displayed nothing. I then reverted to CiviCRM 4.7.20 and a database backup. As far as I can tell there are no other issues other than the mailing scheduler failing. Any help would be appreciated.