Hi there I use The GK_John- template for years on that site.
I have had a lot of problems updating from J5 to J6. wheneveer i checked for updates via the joomla installer and I thought i disabled or uninstalled all old extension and then disabled the J5 plg_behaviour_compat the whole site and administration crashed due to a missing class "Class "J Plugin" not found"
I solved the crashed site or administration by enabling the plugin in the database directly in the extension table. After that delete the plugins cache. delete administrator/cache/com_plugins/filename... Then the backend should work again.

after a while i found out that GK Menu Extended Parameters causes the error (setting error display to maximum see result at the end of post)
Disabling the plugin and then restarting the update process solved the problem for me.

Error ClassNotFoundError
HTTP 500 Whoops, looks like something went wrong.
Attempted to load class "JURI" from the global namespace.
Did you forget a "use" statement?
[2/2] ClassNotFoundError

Symfony\Component\ErrorHandler\Error\ClassNotFoundError:
Attempted to load class "JURI" from the global namespace.
Did you forget a "use" statement?

at /html/J5-koepf/templates/gk_john_s/error.php:18
at require()
(/html/J5-koepf/libraries/src/Document/HtmlDocument.php:734)
at Joomla\CMS\Document\HtmlDocument->loadTemplate('/html/J5-koepf/templates/gk_john_s', 'error.php')
(/html/J5-koepf/libraries/src/Document/HtmlDocument.php:788)
at Joomla\CMS\Document\HtmlDocument->fetchTemplate(array('template' => 'gk_john_s', 'directory' => '/html/J5-koepf/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
(/html/J5-koepf/libraries/src/Document/HtmlDocument.php:625)
at Joomla\CMS\Document\HtmlDocument->parse(array('template' => 'gk_john_s', 'directory' => '/html/J5-koepf/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
(/html/J5-koepf/libraries/src/Document/HtmlDocument.php:643)
at Joomla\CMS\Document\HtmlDocument->render(false, array('template' => 'gk_john_s', 'directory' => '/html/J5-koepf/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
(/html/J5-koepf/libraries/src/Document/ErrorDocument.php:139)
at Joomla\CMS\Document\ErrorDocument->render(false, array('template' => 'gk_john_s', 'directory' => '/html/J5-koepf/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
(/html/J5-koepf/libraries/src/Error/Renderer/HtmlRenderer.php:70)
at Joomla\CMS\Error\Renderer\HtmlRenderer->render(object(Error))
(/html/J5-koepf/libraries/src/Exception/ExceptionHandler.php:142)
at Joomla\CMS\Exception\ExceptionHandler::render(object(Error))
(/html/J5-koepf/libraries/src/Exception/ExceptionHandler.php:76)
at Joomla\CMS\Exception\ExceptionHandler::handleException(object(Error))
(/html/J5-koepf/libraries/src/Application/CMSApplication.php:334)
at Joomla\CMS\Application\CMSApplication->execute()
(/html/J5-koepf/includes/app.php:58)
at require_once('/html/J5-koepf/includes/app.php')
(/html/J5-koepf/index.php:51)

[1/2] Error

Error:
Class "JPlugin" not found

at /html/J5-koepf/plugins/system/gkextmenu/gkextmenu.php:29
at require_once()
(/html/J5-koepf/libraries/src/Extension/ExtensionManagerTrait.php:217)
at Joomla\CMS\Application\CMSApplication->loadPluginFromFilesystem('gkextmenu', 'system')
(/html/J5-koepf/libraries/src/Extension/ExtensionManagerTrait.php:160)
at Joomla\CMS\Application\CMSApplication->loadExtension('Joomla\CMS\Extension\PluginInterface', 'gkextmenu:system', '/html/J5-koepf/plugins/system/gkextmenu')
(/html/J5-koepf/libraries/src/Extension/ExtensionManagerTrait.php:99)
at Joomla\CMS\Application\CMSApplication->bootPlugin('gkextmenu', 'system')
(/html/J5-koepf/libraries/src/Plugin/PluginHelper.php:232)
at Joomla\CMS\Plugin\PluginHelper::import(object(stdClass), true, object(Dispatcher))
(/html/J5-koepf/libraries/src/Plugin/PluginHelper.php:192)
at Joomla\CMS\Plugin\PluginHelper::importPlugin('system', null, true, object(Dispatcher))
(/html/J5-koepf/libraries/src/Application/CMSApplication.php:812)
at Joomla\CMS\Application\CMSApplication->initialiseApp(array('language' => 'en-GB'))
(/html/J5-koepf/libraries/src/Application/SiteApplication.php:638)
at Joomla\CMS\Application\SiteApplication->initialiseApp()
(/html/J5-koepf/libraries/src/Application/SiteApplication.php:237)
at Joomla\CMS\Application\SiteApplication->doExecute()
(/html/J5-koepf/libraries/src/Application/CMSApplication.php:304)
at Joomla\CMS\Application\CMSApplication->execute()
(/html/J5-koepf/includes/app.php:58)
at require_once('/html/J5-koepf/includes/app.php')
(/html/J5-koepf/index.php:51)