Steps to reproduce the issue

I can only reproduce this on the Server of my hoster (alfahosting) which uses PHP 7.4.3 (more info about the PHP Version at https://pastebin.com/yLLBY6ZA ). Both, Ver. 4.1.0 and 4.0.6 showed the same behaviour. On my local setup (PHP 8) everything works fine.

• Upload the joomla files to the hoster
• Go through the installer (works flawlessly, entries in the DB are written)
• After installation go to backend or frontend

Expected result

Seeing the backend/frontend

Actual result

Red error page.
After turning on debug you get the stacktrace:

Class 'Joomla\Plugin\System\Debug\Storage\FileStorage' not found
Call stack
--
# | Function | Location
1 | () | JROOT/plugins/system/debug/debug.php:175
2 | PlgSystemDebug->__construct() | JROOT/libraries/src/Extension/ExtensionManagerTrait.php:242
3 | Joomla\CMS\Application\CMSApplication->loadPluginFromFilesystem() | JROOT/libraries/src/Extension/ExtensionManagerTrait.php:160
4 | Joomla\CMS\Application\CMSApplication->loadExtension() | JROOT/libraries/src/Extension/ExtensionManagerTrait.php:94
5 | Joomla\CMS\Application\CMSApplication->bootPlugin() | JROOT/libraries/src/Plugin/PluginHelper.php:236
6 | Joomla\CMS\Plugin\PluginHelper::import() | JROOT/libraries/src/Plugin/PluginHelper.php:194
7 | Joomla\CMS\Plugin\PluginHelper::importPlugin() | JROOT/libraries/src/Application/CMSApplication.php:742
8 | Joomla\CMS\Application\CMSApplication->initialiseApp() | JROOT/libraries/src/Application/SiteApplication.php:707
9 | Joomla\CMS\Application\SiteApplication->initialiseApp() | JROOT/libraries/src/Application/SiteApplication.php:222
10 | Joomla\CMS\Application\SiteApplication->doExecute() | JROOT/libraries/src/Application/CMSApplication.php:278
11 | Joomla\CMS\Application\CMSApplication->execute() | JROOT/includes/app.php:63
12 | require_once() | JROOT/index.php:32

System information (as much as possible)

PHP 7.4.3, MySQL Database
additional information: https://pastebin.com/yLLBY6ZA
I had the exact same issues with Joomla 4.1.0 and 4.0.6

Additional comments

• I checked that file permissions are correct
• I checked that the corresponding files are not corrupted
• In ExtensionManagerTrait.php - loadPluginFromFilesystem(), I added printouts from the $path variable to verify the pathes to the files are ok - they are
• I checked in debug.php by adding a get_included_files() that the expected files are included
• I used class_exists("FileStorage") in debug.php to check if it finds the class, but it does not
• I tried to change in line 175 in debug.php: FileStorage($storagePath) to Joomla\Plugin\System\Debug\Storage\FileStorage($storagePath) -> no change

Any hint is welcome to trace that down.