I think we can learn a lot just by posting our logged issues. Please including your presta version, PHP version and which log file the issue was retrieved. I only posted each specific line. No need to post double or triple errors.
Presta v1.7.7.8
PHP 7.4 (ea-php74)
I specified the location of the files. This way there is no confusion which error log it was found.
Notice time stamps between UTC and US/Eastern. As my time flips back and fourth? Strange but why?
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Presta >> Backoffice >> Advanced Parameters >> Logs
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Frontcontroller::init - Cart cannot be loaded or an order has already been placed using this cart
/home/xxx/public_html/error_log
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[14-Oct-2021 06:33:35 US/Eastern] PHP Fatal error: Uncaught --> Smarty: 0():Missing '$template' parameter <--
/home/xxx/public_html/Admin/error_log
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[09-Oct-2021 07:35:20 UTC] PHP Deprecated: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead in /home/xxx/public_html/src/Adapter/EntityMapper.php on line 99
[09-Oct-2021 03:35:20 US/Eastern] PHP Deprecated: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead in /home/xxx/public_html/src/Adapter/EntityMapper.php on line 85
[09-Oct-2021 03:35:20 US/Eastern] PHP Deprecated: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead in /home/xxx/public_html/classes/module/Module.php on line 343
[09-Oct-2021 03:35:20 US/Eastern] PHP Deprecated: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead in /home/xxx/public_html/src/Adapter/EntityMapper.php on line 99
[14-Oct-2021 08:31:30 US/Eastern] PHP Warning: count(): Parameter must be an array or an object that implements Countable in /home/xxx/public_html/controllers/admin/AdminCustomerThreadsController.php on line 1021
/home/xxx/public_html/Admin/autoupgrade/tmp/log.txt
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[2021-10-07 12:25:21] [INTERNAL] /home/xxx/public_html/src/Adapter/EntityMapper.php line 99 - array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead