Steps to reproduce the issue

Start with Beta6-dev of 16 December with Blog Sample Data.
The only additional extension is Joomla! Patch Tester, but no patches are currently applied.
Download Joomla_4.0.0-beta6-dev-Development-Update_Package.zip from Nightly Builds.
Run update from System - Update - Joomla - Upload & Update

Expected result

Joomla update is completed.

Actual result

An error has occurred.
0 Class 'Joomla\Component\Joomlaupdate\Administrator\Controller\JFactory' not found

Call stack

Function Location

1 () JROOT\administrator\components\com_joomlaupdate\src\Controller\UpdateController.php:122
2 Joomla\Component\Joomlaupdate\Administrator\Controller\UpdateController->install() JROOT\libraries\src\MVC\Controller\BaseController.php:729
3 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT\libraries\src\Dispatcher\ComponentDispatcher.php:146
4 Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() JROOT\libraries\src\Component\ComponentHelper.php:389
5 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT\libraries\src\Application\AdministratorApplication.php:142
6 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT\libraries\src\Application\AdministratorApplication.php:185
7 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT\libraries\src\Application\CMSApplication.php:266
8 Joomla\CMS\Application\CMSApplication->execute() JROOT\administrator\includes\app.php:63
9 require_once() JROOT\administrator\index.php:36

System information (as much as possible)

Windows 10, Wampserver 3.2.4
Apache 2.4.46a, MySQL 8.0.22, PHP 8.0.0 or PHP 7.4.13

Additional comments

Joomla update worked all right on 16 December. Now the error occurs straight away. Otherwise the Beta6-dev test site is working normally.