The upgrade of CiviCRM 5.12.0 version breaks the user registration process on Drupal 8 : Error type: Could not find a valid session key.
I tried the upgrade on two different D8 sites with same issue.
Select another Civi profile doesn't change it neither.
Apr 06 22:50:18 [info] $backTrace = #0 /server_path/vendor/civicrm/civicrm-core/CRM/Core/Error.php(381): CRM_Core_Error::backtrace("backTrace", TRUE)
#1 /server_path/vendor/civicrm/civicrm-core/CRM/Core/Controller.php(833): CRM_Core_Error::fatal("We can't load the requested web page. This page requires cookies to be enable...")
#2 /server_path/vendor/civicrm/civicrm-core/CRM/Core/Controller.php(828): CRM_Core_Controller->invalidKeyCommon()
#3 /server_path/vendor/civicrm/civicrm-core/CRM/Core/Controller.php(312): CRM_Core_Controller->invalidKey()
#4 /server_path/vendor/civicrm/civicrm-core/CRM/Core/Controller.php(204): CRM_Core_Controller->key("CRM_Profile_Form_Dynamic", TRUE, FALSE)
#5 /server_path/vendor/civicrm/civicrm-core/CRM/Core/Controller/Simple.php(66): CRM_Core_Controller->__construct("Dynamic Form Creator", TRUE, NULL, "CRM_Profile_Form_Dynamic", TRUE, FALSE)
#6 /server_path/vendor/civicrm/civicrm-core/CRM/Core/BAO/UFGroup.php(834): CRM_Core_Controller_Simple->__construct("CRM_Profile_Form_Dynamic", "Dynamic Form Creator", NULL)
#7 /server_path/docroot/modules/contrib/civicrm/civicrm.module(265): CRM_Core_BAO_UFGroup::getEditHTML(NULL, "", NULL, TRUE, TRUE, NULL, FALSE, "Individual")
#8 /server_path/docroot/core/lib/Drupal/Core/Extension/ModuleHandler.php(539): civicrm_form_user_register_form_alter((Array:26), Object(Drupal\Core\Form\FormState), "user_register_form")
#9 /server_path/docroot/core/lib/Drupal/Core/Form/FormBuilder.php(834): Drupal\Core\Extension\ModuleHandler->alter("form", (Array:26), Object(Drupal\Core\Form\FormState), "user_register_form")
#10 /server_path/docroot/core/lib/Drupal/Core/Form/FormBuilder.php(276): Drupal\Core\Form\FormBuilder->prepareForm("user_register_form", (Array:26), Object(Drupal\Core\Form\FormState))
#11 /server_path/docroot/core/lib/Drupal/Core/Controller/FormController.php(93): Drupal\Core\Form\FormBuilder->buildForm("user_register_form", Object(Drupal\Core\Form\FormState))
#12 [internal function](): Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch))
#13 /server_path/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array((Array:2), (Array:2))
#14 /server_path/docroot/core/lib/Drupal/Core/Render/Renderer.php(582): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#15 /server_path/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#16 /server_path/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext((Array:2), (Array:2))
#17 /server_path/vendor/symfony/http-kernel/HttpKernel.php(151): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#18 /server_path/vendor/symfony/http-kernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#19 /server_path/docroot/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#20 /server_path/docroot/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#21 /server_path/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(99): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#22 /server_path/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(78): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#23 /server_path/docroot/core/modules/ban/src/BanMiddleware.php(50): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#24 /server_path/docroot/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\ban\BanMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#25 /server_path/docroot/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#26 /server_path/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#27 /server_path/docroot/core/lib/Drupal/Core/DrupalKernel.php(693): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, TRUE)
#28 /server_path/docroot/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#29 {main}