500 Errors on Drupal 8.9+ and 9+

Problem/Motivation

After a fresh install of latest Drupal Commerce modules whenever I try to edit the default checkout flow, I get the following errors

[error] 1072#1072: *1 FastCGI sent in stderr: "PHP message: Uncaught PHP Exception LogicException: "Settings can not be serialized. This probably means you are serializing an object that has an indirect reference to the Settings object. Adjust your code so that is not necessary." at /xxx/xxx/xxx/core/lib/Drupal/Core/Site/Settings.php line 68" while reading response header from upstream, client: 127.0.0.1, server: xxx.xxx, request: "POST /admin/commerce/config/checkout-flows/manage/default?ajax_form=1&_wrapper_format=drupal_ajax HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.3-fpm-one.sock:", host: "xxx.xxx", referrer: "https://xxx.xxx/admin/commerce/config/checkout-flows/manage/default"

In console, I keep getting these JS errors

Failed to load resource: the server responded with a status of 500 (500 Service unavailable (with message))

Uncaught 
Drupal.AjaxError
message: "↵An AJAX HTTP error occurred.↵HTTP Result Code: 500↵Debugging information follows.↵Path: /admin/commerce/config/checkout-flows/manage/default?destination=/admin/commerce/config/checkout-flows&ajax_form=1↵StatusText: 500 Service unavailable (with message)↵ResponseText: The website encountered an unexpected error. Please try again later.LogicException: Settings can not be serialized. This probably means you are serializing an object that has an indirect reference to the Settings object. Adjust your code so that is not necessary. in Drupal\Core\Site\Settings->__sleep() (line 68 of core/lib/Drupal/Core/Site/Settings.php). serialize(Array) (Line: 14)↵Drupal\Component\Serialization\PhpSerialize::encode(Array) (Line: 80)↵Drupal\Core\KeyValueStore\DatabaseStorageExpirable->setWithExpire('form-N54MTRLssndXf26RMLAQGrjGs2yHnPM0R-ZfF0Xq7SU', Array, 21600) (Line: 197)↵Drupal\Core\Form\FormCache->setCache('form-N54MTRLssndXf26RMLAQGrjGs2yHnPM0R-ZfF0Xq7SU', Array, Object) (Line: 459)↵Drupal\Core\Form\FormBuilder->setCache('form-N54MTRLssndXf26RMLAQGrjGs2yHnPM0R-ZfF0Xq7SU', Array, Object) (Line: 437)↵Drupal\Core\Form\FormBuilder->rebuildForm('commerce_checkout_flow_edit_form', Object, Array) (Line: 629)↵Drupal\Core\Form\FormBuilder->processForm('commerce_checkout_flow_edit_form', Array, Object) (Line: 321)↵Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 91)↵Drupal\Core\Controller\FormController->getContentResult(Object, Object)↵call_user_func_array(Array, Array) (Line: 123)↵Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)↵Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)↵Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)↵Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)↵Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)↵Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)↵Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)↵Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)↵Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)↵Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)↵Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)↵Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)↵Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)↵Drupal\Core\DrupalKernel->handle(Object) (Line: 19)↵"
name: "AjaxError"
__proto__: Error at /assets/files/js/js_TdiMRR3BXgOgIRoGh26DZjaqiPeXu2B4dXeyhxcJsws.js:1697:32 at /assets/files/js/js_TdiMRR3BXgOgIRoGh26DZjaqiPeXu2B4dXeyhxcJsws.js:2240:3

Steps to reproduce

Install Drupal commence 2.20 on a fresh install of Drupal core and then edit or create a new commerce checkout flow.

I do have commerce installations on the same server, which do not suffer from the same issue. I tried to upgrade to Dev version or downgrade to 2.19, however, the error is persistent.