Symptom: you are getting debug logs like this:

[21-Oct-2020 08:17:36 UTC] Request URI: /index.php?main_page=product_info&products_id=1234
#1 require_once(/includes/classes/zen_lightbox/options.php) called at [/includes/classes/zen_lightbox/autoload_default.php:14]
#2 require_once(/includes/classes/zen_lightbox/autoload_default.php) called at [/includes/modules/pages/product_info/jscript_zen_lightbox.php:14]
#3 require(/includes/modules/pages/product_info/jscript_zen_lightbox.php) called at [/includes/templates/custom_responsive/common/html_header.php:179]
#4 require(/includes/templates/custom_responsive/common/html_header.php) called at [/index.php:42]
--> PHP Warning: Use of undefined constant ZEN_LIGHTBOX_COUNTER_IMAGE - assumed 'ZEN_LIGHTBOX_COUNTER_IMAGE' (this will throw an Error in a future version of PHP) in /includes/classes/zen_lightbox/options.php on line 23.

Solution:

Create or append to includes/languages/english/extra_definitions/zen_lightbox.php

PHP Code:
define('ZEN_LIGHTBOX_COUNTER_IMAGE', 'Image');
define('ZEN_LIGHTBOX_COUNTER_OF', 'of');