User error: "attributes" is an invalid render array key in Drupal\Core\Render\Element::children()

Repeated error on all page loads:

User error: "attributes" is an invalid render array key in Drupal\Core\Render\Element::children() (line 93 of /path/to/drupal/core/lib/Drupal/Core/Render/Element.php).

I assume this is template related. If you give suggestions, I would be happy to do some editing and testing.