"Error 503 – Service Unavailable" showing instead of Maintenance Mode page

I just upgraded to Drupal core 7.58.  When Maintenance Mode is enabled, I’m now getting an “Error 503 – Service Unavailable” page instead of the Maintenance Mode message page.  I’m not 100% sure if this was happening before 7.58, but pretty sure it was fine.

This works fine in my local install, but not on the production site (hosted on a reseller plan at HostGator – Linux, PHP 5.4.45, MySQL 5.5.51-38.2).  I can log in just fine on the production site, so it’s just the maintenance mode page that’s not showing anymore.  Clean URLs are working, no message about this in the logs, etc.

Any ideas?  Thanks in advance for the help.

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/22/feed