The specified file 'temporary://xxxxx' could not be copied but only on Pantheon

Problem/Motivation

I get WSOD displaying message "The website encountered an unexpected error. Try again later." When I try to save configuration on my sub-theme

I tried working with Pantheon on this issue and they told me that the destination folder is not setup to be written to. They suggested I setup a symlink to move the destination folder to a writable location (web/sites/default/files) but doing that did not work and it also broke the rest of the css.

Steps to reproduce

Go into the sub-theme and try doing a 'save config'

The website location is https://updates-lflb-history.pantheonsite.io

The error output is

Type php
Location https://updates-lflb-history.pantheonsite.io/admin/appearance/settings/l...
Referrer https://updates-lflb-history.pantheonsite.io/admin/appearance/settings/l...

I get WSOD displaying message "The website encountered an unexpected error. Try again later." When I try to save configuration on my sub-theme

Drupal\Core\File\Exception\DirectoryNotReadyException: The specified file 'temporary://file1NtrpH' could not be copied because the destination directory 'themes/custom/lflb_history/styles/css/generated' is not properly configured. This may be caused by a problem with file or directory permissions. in Drupal\Core\File\FileSystem->prepareDestination() (line 457 of /code/web/core/lib/Drupal/Core/File/FileSystem.php).

Severity Error

#0 /code/web/core/lib/Drupal/Core/File/FileSystem.php(376): Drupal\Core\File\FileSystem->prepareDestination('temporary://fil...', 'themes/custom/l...', Object(Drupal\Core\File\FileExists))
#1 /code/web/core/lib/Drupal/Core/File/FileSystem.php(490): Drupal\Core\File\FileSystem->move('temporary://fil...', 'themes/custom/l...', Object(Drupal\Core\File\FileExists))
#2 /code/web/themes/contrib/adaptivetheme/at_core/forms/ext/fonts_submit.php(187): Drupal\Core\File\FileSystem->saveData('html, body, tex...', 'themes/custom/l...', Object(Drupal\Core\File\FileExists))
#3 /code/web/themes/contrib/adaptivetheme/at_core/forms/ext/extension_settings_submit.php(42): at_core_submit_fonts(Array, 'themes/custom/l...')
#4 [internal function]: at_core_submit_extension_settings(Array, Object(Drupal\Core\Form\FormState))
#5 /code/web/core/lib/Drupal/Core/Form/FormSubmitter.php(129): call_user_func_array('at_core_submit_...', Array)
#6 /code/web/core/lib/Drupal/Core/Form/FormSubmitter.php(67): Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object(Drupal\Core\Form\FormState))
#7 /code/web/core/lib/Drupal/Core/Form/FormBuilder.php(597): Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object(Drupal\Core\Form\FormState))
#8 /code/web/core/lib/Drupal/Core/Form/FormBuilder.php(326): Drupal\Core\Form\FormBuilder->processForm('system_theme_se...', Array, Object(Drupal\Core\Form\FormState))
#9 /code/web/core/lib/Drupal/Core/Controller/FormController.php(73): Drupal\Core\Form\FormBuilder->buildForm(Object(Drupal\system\Form\ThemeSettingsForm), Object(Drupal\Core\Form\FormState))
#10 [internal function]: Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch))
#11 /code/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array)
#12 /code/web/core/lib/Drupal/Core/Render/Renderer.php(638): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#13 /code/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(121): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#14 /code/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#15 /code/vendor/symfony/http-kernel/HttpKernel.php(181): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#16 /code/vendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#17 /code/web/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#18 /code/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#19 /code/web/core/lib/Drupal/Core/StackMiddleware/ContentLength.php(28): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#20 /code/web/core/modules/big_pipe/src/StackMiddleware/ContentLength.php(32): Drupal\Core\StackMiddleware\ContentLength->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#21 /code/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(116): Drupal\big_pipe\StackMiddleware\ContentLength->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#22 /code/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(90): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#23 /code/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#24 /code/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#25 /code/web/core/lib/Drupal/Core/StackMiddleware/AjaxPageState.php(36): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#26 /code/web/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\AjaxPageState->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#27 /code/web/core/lib/Drupal/Core/DrupalKernel.php(741): Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#28 /code/web/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#29 {main}

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes