I don't understand if the solution is downgrading, or if something else was done to solve. In any case, I have the same issue on a completely new plain vanilla installation. I have done nothing but set configurations exactly as described in the https://docs.magento.com/m2/ce/user_guide Here is the extract from the system.log:
[2018-07-06 17:09:19] main.CRITICAL: Exception message: Area is already set
Trace: #0 /blahblah/magento/generated/code/Magento/Email/Model/Template/Interceptor.php(284): Magento\Email\Model\AbstractTemplate->setForcedArea('design_email_he...')
#1 /blahblah/magento/vendor/magento/module-theme/Model/Design/Config/Validator.php(117): Magento\Email\Model\Template\Interceptor->setForcedArea('design_email_he...')
#2 /blahblah/magento/vendor/magento/module-theme/Model/Design/Config/Validator.php(68): Magento\Theme\Model\Design\Config\Validator->getTemplateText('design_email_he...', Object(Magento\Theme\Model\Data\Design\Config))
#3 /blahblah/magento/vendor/magento/module-theme/Model/DesignConfigRepository.php(91): Magento\Theme\Model\Design\Config\Validator->validate(Object(Magento\Theme\Model\Data\Design\Config))
#4 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Theme\Model\DesignConfigRepository->save(Object(Magento\Theme\Model\Data\Design\Config))
#5 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Theme\Model\DesignConfigRepository\Interceptor->___callParent('save', Array)
#6 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(153): Magento\Theme\Model\DesignConfigRepository\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Theme\Model\Data\Design\Config))
#7 /blahblah/magento/generated/code/Magento/Theme/Model/DesignConfigRepository/Interceptor.php(39): Magento\Theme\Model\DesignConfigRepository\Interceptor->___callPlugins('save', Array, Array)
#8 /blahblah/magento/vendor/magento/module-theme/Controller/Adminhtml/Design/Config/Save.php(79): Magento\Theme\Model\DesignConfigRepository\Interceptor->save(Object(Magento\Theme\Model\Data\Design\Config))
#9 /blahblah/magento/generated/code/Magento/Theme/Controller/Adminhtml/Design/Config/Save/Interceptor.php(24): Magento\Theme\Controller\Adminhtml\Design\Config\Save->execute()
#10 /blahblah/magento/vendor/magento/framework/App/Action/Action.php(107): Magento\Theme\Controller\Adminhtml\Design\Config\Save\Interceptor->execute()
#11 /blahblah/magento/vendor/magento/module-backend/App/AbstractAction.php(229): Magento\Framework\App\Action\Action->dispatch(Object(Magento\Framework\App\Request\Http))
#12 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Backend\App\AbstractAction->dispatch(Object(Magento\Framework\App\Request\Http))
#13 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Theme\Controller\Adminhtml\Design\Config\Save\Interceptor->___callParent('dispatch', Array)
#14 /blahblah/magento/vendor/magento/module-backend/App/Action/Plugin/Authentication.php(143): Magento\Theme\Controller\Adminhtml\Design\Config\Save\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http))
#15 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(135): Magento\Backend\App\Action\Plugin\Authentication->aroundDispatch(Object(Magento\Theme\Controller\Adminhtml\Design\Config\Save\Interceptor), Object(Closure), Object(Magento\Framework\App\Request\Http))
#16 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(153): Magento\Theme\Controller\Adminhtml\Design\Config\Save\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http))
#17 /blahblah/magento/generated/code/Magento/Theme/Controller/Adminhtml/Design/Config/Save/Interceptor.php(39): Magento\Theme\Controller\Adminhtml\Design\Config\Save\Interceptor->___callPlugins('dispatch', Array, NULL)
#18 /blahblah/magento/vendor/magento/framework/App/FrontController.php(55): Magento\Theme\Controller\Adminhtml\Design\Config\Save\Interceptor->dispatch(Object(Magento\Framework\App\Request\Http))
#19 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Framework\App\FrontController->dispatch(Object(Magento\Framework\App\Request\Http))
#20 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Framework\App\FrontController\Interceptor->___callParent('dispatch', Array)
#21 /blahblah/magento/vendor/magento/framework/Interception/Interceptor.php(153): Magento\Framework\App\FrontController\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http))
#22 /blahblah/magento/generated/code/Magento/Framework/App/FrontController/Interceptor.php(26): Magento\Framework\App\FrontController\Interceptor->___callPlugins('dispatch', Array, Array)
#23 /blahblah/magento/vendor/magento/framework/App/Http.php(135): Magento\Framework\App\FrontController\Interceptor->dispatch(Object(Magento\Framework\App\Request\Http))
#24 /blahblah/magento/generated/code/Magento/Framework/App/Http/Interceptor.php(24): Magento\Framework\App\Http->launch()
#25 /blahblah/magento/vendor/magento/framework/App/Bootstrap.php(256): Magento\Framework\App\Http\Interceptor->launch()
#26 /blahblah/magento/index.php(39): Magento\Framework\App\Bootstrap->run(Object(Magento\Framework\App\Http\Interceptor))
#27 {main} [] []
