Website was migrated from Joomla3.10. We cannot reproduce the issue under fresh Joomla4 installs. Could it be that old php code is touched on the update?

@ariesmitskamp If you switch on "Debug System" in Global Configuration in backend then you would get a stack trace with this error. Can you do that and report back? We could know then if it is one of the PHP 8 issues which we already fixed and which will be available with the next 4.0.6 release, or if it is something different and if so, if we already have a report for that or not.

Thanks in advance.

Hi @richard67 hereby the stack trace. It could be that a 3rd party plugin is the problem. If so, sorry for disturbing you. Just want to help in testing and this is the first time in 12 years Joomla that I report an issue...

`Call stack

Function Location

1 () JROOT/plugins/system/sywautoreset/sywautoreset.php:205
2 array_count_values() JROOT/plugins/system/sywautoreset/sywautoreset.php:205
3 plgSystemSYWAutoReset->onContentBeforeSave() JROOT/libraries/src/Plugin/CMSPlugin.php:285
4 Joomla\CMS\Plugin\CMSPlugin->Joomla\CMS\Plugin{closure}() JROOT/libraries/vendor/joomla/event/src/Dispatcher.php:486
5 Joomla\Event\Dispatcher->dispatch() JROOT/libraries/src/Application/EventAware.php:111
6 Joomla\CMS\Application\WebApplication->triggerEvent() JROOT/libraries/src/MVC/Model/AdminModel.php:1391
7 Joomla\CMS\MVC\Model\AdminModel->save() JROOT/administrator/components/com_content/src/Model/ArticleModel.php:832
8 Joomla\Component\Content\Administrator\Model\ArticleModel->save() JROOT/libraries/src/MVC/Controller/FormController.php:705
9 Joomla\CMS\MVC\Controller\FormController->save() JROOT/libraries/src/MVC/Controller/BaseController.php:730
10 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/libraries/src/Dispatcher/ComponentDispatcher.php:146
11 Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:389
12 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:143
13 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:186
14 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:278
15 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/includes/app.php:63
16 require_once() JROOT/administrator/index.php:32`

If you unpublish disable the system plugin "sywautoreset", does the error still happen?

No it doesn't, so this was a false alarm sorry. Thank you for your help. I'll report it to the extension developer.

Closing as not a core issue.