Brochure site fails repeatedly and randomly: "line 49 of core/modules/views/src/Element/View.php"

My site, which is essentially “read only,” randomly fails to load, with the following message:

The website encountered an unexpected error. Please try again later.
Error: Unsupported operand types in DrupalviewsElementView::preRenderViewElement() (line 49 of core/modules/views/src/Element/View.php).

Here are the environment specifics: 
  Hosting environment: Pantheon (Personal service level)
  Drupal: 8.5.3
  PHP: 7.1.17
  DB System Version: 5.5.5-10.0.23-MariaDB-log

I’ve been chasing this ever since I moved the site to Pantheon. It’s persisted through a couple of Core updates. The problem didn’t occur on my Aquia DevDesktop environment, or when I had it hosted on Gandi.  

When the problem occurs (which is now daily), I’m able to restore functinality by cloning the db from Dev or Test.  Restoring from Backup doesn’t correct the problem.

Line 49 of View.php is: $element += $view->element;

I’ve been a db developer for 30+ years, but I’m a total newbie to Drupal and Pantheon.  I’m not even sure where to start in diagnosing this problem.  I’ll be grateful for any suggestions. 
 

Drupal version: 


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