OK, I found what looks to be a logfile tied to the error. I've tweaked it just to hide the admin directory name. Looks like something to do with checking a version number?
[03-May-2018 20:40:54 America/Los_Angeles] Request URI: /shop/...../modules.php?set=payment&module=square, IP address: 71.95.106.83
#1 trigger_error() called at [C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php:68]
#2 plugin_version_check_for_updates() called at [C:\Web Files\localuser\CHCG\Shop\includes\modules\payment\square.php:87]
#3 square->__construct() called at [C:\Web Files\localuser\CHCG\Shop.....\modules.php:185]
[03-May-2018 20:40:54 America/Los_Angeles] PHP Notice: CURL error checking plugin versions: 60:SSL certificate problem: self signed certificate in certificate chain
Trying file_get_contents() instead. in C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php on line 68
[03-May-2018 20:40:56 America/Los_Angeles] Request URI: /shop/...../modules.php?set=payment&module=square, IP address: 71.95.106.83
#1 file_get_contents() called at [C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php:70]
#2 plugin_version_check_for_updates() called at [C:\Web Files\localuser\CHCG\Shop\includes\modules\payment\square.php:87]
#3 square->__construct() called at [C:\Web Files\localuser\CHCG\Shop.....\modules.php:185]
[03-May-2018 20:40:56 America/Los_Angeles] PHP Warning: file_get_contents(https://plugins.zen-cart.com/versioncheck/156%29: failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request
in C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php on line 70
[03-May-2018 20:40:56 America/Los_Angeles] Request URI: /shop/...../modules.php?set=payment&module=square, IP address: 71.95.106.83
#1 trigger_error() called at [C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php:72]
#2 plugin_version_check_for_updates() called at [C:\Web Files\localuser\CHCG\Shop\includes\modules\payment\square.php:87]
#3 square->__construct() called at [C:\Web Files\localuser\CHCG\Shop.....\modules.php:185]
[03-May-2018 20:40:56 America/Los_Angeles] PHP Notice: file_get_contents() error checking plugin versions.
Trying http instead. in C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php on line 72
[03-May-2018 20:40:56 America/Los_Angeles] Request URI: /shop/...../modules.php?set=payment&module=square, IP address: 71.95.106.83
#1 file_get_contents() called at [C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php:73]
#2 plugin_version_check_for_updates() called at [C:\Web Files\localuser\CHCG\Shop\includes\modules\payment\square.php:87]
#3 square->__construct() called at [C:\Web Files\localuser\CHCG\Shop.....\modules.php:185]
[03-May-2018 20:40:56 America/Los_Angeles] PHP Warning: file_get_contents(http://plugins.zen-cart.com/versioncheck/156%29: failed to open stream: HTTP request failed! HTTP/1.0 404 Not Found
in C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php on line 73
[03-May-2018 20:40:56 America/Los_Angeles] Request URI: /shop/...../modules.php?set=payment&module=square, IP address: 71.95.106.83
#1 trigger_error() called at [C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php:76]
#2 plugin_version_check_for_updates() called at [C:\Web Files\localuser\CHCG\Shop\includes\modules\payment\square.php:87]
#3 square->__construct() called at [C:\Web Files\localuser\CHCG\Shop.....\modules.php:185]
[03-May-2018 20:40:56 America/Los_Angeles] PHP Notice: file_get_contents() error checking plugin versions.
Aborting. in C:\Web Files\localuser\CHCG\Shop\includes\functions\plugin_support.php on line 76