Steps to reproduce the issue

Download Beta8-dev full package of 29 March: https://developer.joomla.org/nightlies/Joomla_4.0.0-beta8-dev-Development-Full_Package.zip
Expand and install.
Remove the installation folder.
Rename htaccess.txt to .htaccess.
Go to the site and the back end.

Expected result

No errors displayed.

Actual result

Fatal error: Trait "Joomla\Plugin\System\Webauthn\PluginTraits\AjaxHandler" not found in C:\www\joomla4\plugins\system\webauthn\webauthn.php on line 34

System information (as much as possible)

Windows 10 with Wampserver 3.2.4
Apache 2.4.36c
MySQL 8.0.23
PHP 8.0.2

Additional comments

Stack trace from back end

[29-Mar-2021 09:21:54 UTC] PHP Fatal error:  Trait "Joomla\Plugin\System\Webauthn\PluginTraits\AjaxHandler" not found in C:\www\joomla4\plugins\system\webauthn\webauthn.php on line 34
[29-Mar-2021 09:21:54 UTC] PHP Stack trace:
[29-Mar-2021 09:21:54 UTC] PHP   1. {main}() C:\www\joomla4\administrator\index.php:0
[29-Mar-2021 09:21:54 UTC] PHP   2. require_once() C:\www\joomla4\administrator\index.php:32
[29-Mar-2021 09:21:54 UTC] PHP   3. Joomla\CMS\Application\AdministratorApplication->execute() C:\www\joomla4\administrator\includes\app.php:63
[29-Mar-2021 09:21:54 UTC] PHP   4. Joomla\CMS\Application\AdministratorApplication->doExecute() C:\www\joomla4\libraries\src\Application\CMSApplication.php:266
[29-Mar-2021 09:21:54 UTC] PHP   5. Joomla\CMS\Application\AdministratorApplication->initialiseApp($options = ['language' => NULL]) C:\www\joomla4\libraries\src\Application\AdministratorApplication.php:165
[29-Mar-2021 09:21:54 UTC] PHP   6. Joomla\CMS\Application\AdministratorApplication->initialiseApp($options = ['language' => 'en-GB']) C:\www\joomla4\libraries\src\Application\AdministratorApplication.php:354
[29-Mar-2021 09:21:54 UTC] PHP   7. Joomla\CMS\Plugin\PluginHelper::importPlugin($type = 'system', $plugin = *uninitialized*, $autocreate = *uninitialized*, $dispatcher = *uninitialized*) C:\www\joomla4\libraries\src\Application\CMSApplication.php:714
[29-Mar-2021 09:21:54 UTC] PHP   8. Joomla\CMS\Plugin\PluginHelper::import($plugin = class stdClass { public $type = 'system'; public $name = 'webauthn'; public $params = '{}'; public $id = 183 }, $autocreate = TRUE, $dispatcher = class Joomla\Event\Dispatcher { protected $events = []; protected $listeners = ['session.start' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableObjectCreate' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableBeforeStore' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableAfterStore' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableBeforeDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableSetNewTags' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableAfterReset' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableAfterLoad' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onBeforeBatch' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentChangeState' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onApplicationAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterInstall' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterUninstall' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterUpdate' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterSaveGroup' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterDeleteGroup' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterLogin' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserLoginFailure' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserLogout' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterRemind' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterCheckin' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterLogPurge' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterLogExport' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterPurge' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterDispatch' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onJoomlaAfterUpdate' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterInitialise' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentPrepareForm' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentPrepareData' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterRespond' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onBeforeCompileHead' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxDebug' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterDisconnect' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentNormaliseRequestData' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterTitle' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentBeforeDisplay' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterDisplay' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentPrepare' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onPrepareFinderContent' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onFinderResult' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterRender' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserBeforeSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendAlways' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendNever' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendOnce' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendStats' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onGetStatsData' => class Joomla\Event\ListenersPriorityQueue { ... }] }) C:\www\joomla4\libraries\src\Plugin\PluginHelper.php:193
[29-Mar-2021 09:21:54 UTC] PHP   9. Joomla\CMS\Application\AdministratorApplication->bootPlugin($plugin = 'webauthn', $type = 'system') C:\www\joomla4\libraries\src\Plugin\PluginHelper.php:235
[29-Mar-2021 09:21:54 UTC] PHP  10. Joomla\CMS\Application\AdministratorApplication->loadExtension($type = 'Joomla\\CMS\\Extension\\PluginInterface', $extensionName = 'webauthn:system', $extensionPath = 'C:\\www\\joomla4/plugins/system/webauthn') C:\www\joomla4\libraries\src\Extension\ExtensionManagerTrait.php:94
[29-Mar-2021 09:21:54 UTC] PHP  11. Joomla\CMS\Application\AdministratorApplication->loadPluginFromFilesystem($plugin = 'webauthn', $type = 'system') C:\www\joomla4\libraries\src\Extension\ExtensionManagerTrait.php:160
[29-Mar-2021 09:21:54 UTC] PHP  12. require_once() C:\www\joomla4\libraries\src\Extension\ExtensionManagerTrait.php:219

Stack trace from front end

[29-Mar-2021 09:21:54 UTC] PHP Fatal error:  Trait "Joomla\Plugin\System\Webauthn\PluginTraits\AjaxHandler" not found in C:\www\joomla4\plugins\system\webauthn\webauthn.php on line 34
[29-Mar-2021 09:21:54 UTC] PHP Stack trace:
[29-Mar-2021 09:21:54 UTC] PHP   1. {main}() C:\www\joomla4\index.php:0
[29-Mar-2021 09:21:54 UTC] PHP   2. require_once() C:\www\joomla4\index.php:32
[29-Mar-2021 09:21:54 UTC] PHP   3. Joomla\CMS\Application\SiteApplication->execute() C:\www\joomla4\includes\app.php:63
[29-Mar-2021 09:21:54 UTC] PHP   4. Joomla\CMS\Application\SiteApplication->doExecute() C:\www\joomla4\libraries\src\Application\CMSApplication.php:266
[29-Mar-2021 09:21:54 UTC] PHP   5. Joomla\CMS\Application\SiteApplication->initialiseApp($options = *uninitialized*) C:\www\joomla4\libraries\src\Application\SiteApplication.php:224
[29-Mar-2021 09:21:54 UTC] PHP   6. Joomla\CMS\Application\SiteApplication->initialiseApp($options = ['language' => 'en-GB']) C:\www\joomla4\libraries\src\Application\SiteApplication.php:709
[29-Mar-2021 09:21:54 UTC] PHP   7. Joomla\CMS\Plugin\PluginHelper::importPlugin($type = 'system', $plugin = *uninitialized*, $autocreate = *uninitialized*, $dispatcher = *uninitialized*) C:\www\joomla4\libraries\src\Application\CMSApplication.php:714
[29-Mar-2021 09:21:54 UTC] PHP   8. Joomla\CMS\Plugin\PluginHelper::import($plugin = class stdClass { public $type = 'system'; public $name = 'webauthn'; public $params = '{}'; public $id = 183 }, $autocreate = TRUE, $dispatcher = class Joomla\Event\Dispatcher { protected $events = []; protected $listeners = ['session.start' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableObjectCreate' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableBeforeStore' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableAfterStore' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableBeforeDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableSetNewTags' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableAfterReset' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onTableAfterLoad' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onBeforeBatch' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentChangeState' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onApplicationAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterInstall' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterUninstall' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterUpdate' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onExtensionAfterDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterDelete' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterSaveGroup' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterDeleteGroup' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterLogin' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserLoginFailure' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserLogout' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserAfterRemind' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterCheckin' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterLogPurge' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterLogExport' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterPurge' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterDispatch' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onJoomlaAfterUpdate' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterInitialise' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentPrepareForm' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentPrepareData' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterRespond' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onBeforeCompileHead' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxDebug' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterDisconnect' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentNormaliseRequestData' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterTitle' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentBeforeDisplay' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentAfterDisplay' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onContentPrepare' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onPrepareFinderContent' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onFinderResult' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAfterRender' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onUserBeforeSave' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendAlways' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendNever' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendOnce' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onAjaxSendStats' => class Joomla\Event\ListenersPriorityQueue { ... }, 'onGetStatsData' => class Joomla\Event\ListenersPriorityQueue { ... }] }) C:\www\joomla4\libraries\src\Plugin\PluginHelper.php:193
[29-Mar-2021 09:21:54 UTC] PHP   9. Joomla\CMS\Application\SiteApplication->bootPlugin($plugin = 'webauthn', $type = 'system') C:\www\joomla4\libraries\src\Plugin\PluginHelper.php:235
[29-Mar-2021 09:21:54 UTC] PHP  10. Joomla\CMS\Application\SiteApplication->loadExtension($type = 'Joomla\\CMS\\Extension\\PluginInterface', $extensionName = 'webauthn:system', $extensionPath = 'C:\\www\\joomla4/plugins/system/webauthn') C:\www\joomla4\libraries\src\Extension\ExtensionManagerTrait.php:94
[29-Mar-2021 09:21:54 UTC] PHP  11. Joomla\CMS\Application\SiteApplication->loadPluginFromFilesystem($plugin = 'webauthn', $type = 'system') C:\www\joomla4\libraries\src\Extension\ExtensionManagerTrait.php:160
[29-Mar-2021 09:21:54 UTC] PHP  12. require_once() C:\www\joomla4\libraries\src\Extension\ExtensionManagerTrait.php:219