Getting the following error (which was also sometimes get occasionally on some other sites with Core 5.3.3 and previous versions even without rolling back):
Admin Tools - An unhandled Exception has been detected
500 FOF: Model \Akeeba\AdminTools\Admin\Model\ConfigureWAF not found
File <root>/libraries/fof30/Factory/BasicFactory.php Line 554
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Debug information
Exception type: FOF30\Factory\Exception\ModelNotFound
#0 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/fof30/Factory/BasicFactory.php(155): FOF30\Factory\BasicFactory->createModel('\\Akeeba\\AdminTo...', Array)
#1 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/administrator/components/com_admintools/Controller/Mixin/SendTroubleshootingEmail.php(31): FOF30\Factory\BasicFactory->model('ConfigureWAF')
#2 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/administrator/components/com_admintools/Controller/AdminPassword.php(60): Akeeba\AdminTools\Admin\Controller\AdminPassword->sendTroubelshootingEmail('AdminPassword')
#3 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/fof30/Controller/Controller.php(420): Akeeba\AdminTools\Admin\Controller\AdminPassword->protect()
#4 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/administrator/components/com_admintools/Controller/Mixin/PredefinedTaskList.php(40): FOF30\Controller\Controller->execute('protect')
#5 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/fof30/Dispatcher/Dispatcher.php(175): Akeeba\AdminTools\Admin\Controller\AdminPassword->execute('protect')
#6 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/administrator/components/com_admintools/admintools.php(60): FOF30\Dispatcher\Dispatcher->dispatch()
#7 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/administrator/components/com_admintools/admintools.php(90): mainLoopAdminToolsForJoomla()
#8 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/src/Component/ComponentHelper.php(402): require_once('/mnt/stor13-wc2...')
#9 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/src/Component/ComponentHelper.php(377): Joomla\CMS\Component\ComponentHelper::executeComponent('/mnt/stor13-wc2...')
#10 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/src/Application/AdministratorApplication.php(101): Joomla\CMS\Component\ComponentHelper::renderComponent('com_admintools')
#11 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/src/Application/AdministratorApplication.php(159): Joomla\CMS\Application\AdministratorApplication->dispatch()
#12 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/libraries/src/Application/CMSApplication.php(196): Joomla\CMS\Application\AdministratorApplication->doExecute()
#13 /mnt/stor13-wc2-dfw1/561654/2002532/www.twillamedspa.com/web/content/administrator/index.php(51): Joomla\CMS\Application\CMSApplication->execute()
#14 {main}