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When you view the site's Tools->Server/Version information, does it show perhaps that the site is running on PHP 5.6 FPM? That PHP installation doesn't like to give up control as to where the PHP error-logs are written. Within that Server/Version information, find the
error_log setting (which identifies where PHP logs will be written). For the admin, it
should show something like c:/xampp/htdocs/mysite/logs/myDEBUG-adm-1555415275-25535.log (that suffix will change on each page reload).
The error logging for zc156 has been modified to take the FPM installations into account, so you might try using the /includes/extra_configures/enable_error_logging.php (and the associated /admin/includes/extra_configures/enable_error_logging) from Zen Cart's GitHub v156 branch (
https://github.com/zencart/zencart/tree/v156).
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