System information (as much as possible) ?

J 3.9.x or J 4.0 ?

4 beta 7.
PHP 7.4.15

I cannot replicate this error on j4.

Sounds like a sef/htaccess setup issue

SEF set to off, using a vanilla Joomla install, just the way I always start a Joomla site. Never happened before with current/previous versions.

@trogladyte I have no clue what happened, but maybe you could renew your installation? Or can open the console and look for js errors?

Turned on Debug and Maximum error reporting. The 500 pages are giving:

behavior::caption not found.
/home/wwwckde/public_html/iancfi/libraries/src/HTML/HTMLHelper.php:149

Call stack

| Function | Location

1 | () | JROOT/libraries/src/HTML/HTMLHelper.php:149
2 | Joomla\CMS\HTML\HTMLHelper::_() | JROOT/templates/ian-cfi-march-2021/html/com_content/article/default.php:24
3 | include() | JROOT/libraries/src/MVC/View/HtmlView.php:429
4 | Joomla\CMS\MVC\View\HtmlView->loadTemplate() | JROOT/libraries/src/MVC/View/HtmlView.php:218
5 | Joomla\CMS\MVC\View\HtmlView->display() | JROOT/components/com_content/src/View/Article/HtmlView.php:260
6 | Joomla\Component\Content\Site\View\Article\HtmlView->display() | JROOT/libraries/src/MVC/Controller/BaseController.php:692
7 | Joomla\CMS\MVC\Controller\BaseController->display() | JROOT/components/com_content/src/Controller/DisplayController.php:125
8 | Joomla\Component\Content\Site\Controller\DisplayController->display() | JROOT/libraries/src/MVC/Controller/BaseController.php:730
9 | Joomla\CMS\MVC\Controller\BaseController->execute() | JROOT/libraries/src/Dispatcher/ComponentDispatcher.php:146
10 | Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() | JROOT/components/com_content/src/Dispatcher/Dispatcher.php:55
11 | Joomla\Component\Content\Site\Dispatcher\Dispatcher->dispatch() | JROOT/libraries/src/Component/ComponentHelper.php:389
12 | Joomla\CMS\Component\ComponentHelper::renderComponent() | JROOT/libraries/src/Application/SiteApplication.php:206
13 | Joomla\CMS\Application\SiteApplication->dispatch() | JROOT/libraries/src/Application/SiteApplication.php:245
14 | Joomla\CMS\Application\SiteApplication->doExecute() | JROOT/libraries/src/Application/CMSApplication.php:266
15 | Joomla\CMS\Application\CMSApplication->execute() | JROOT/includes/app.php:63
16 | require_once() | JROOT/index.php:32

@trogladyte The problem comes from your override layout JROOT/templates/ian-cfi-march-2021/html/com_content/article/default.php

Remove that override will solve the issue.

Thanks Tuan, and apologies everyone - seems I created the issue! I'll talk with the dev about it.

@trogladyte Just for your information, the error happen because you call JHtml::_('behavior.caption') in the code of that override layout. That method was Deprecated in Joomla 3 and removed in Joomla 4, thus you are getting that error. From backward in-compatible changes documentation https://docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_4#Manual_Include_Behavior_Removed

The caption.js file has been removed from Joomla. Use the native HTML elements figure and figcaption. (You can look at JLayout in layouts/joomla/content/intro_image.php for an example).

As this is not Joomla! issue, please close this issue.

I am getting a similar error:

An error has occurred.
500 behavior::calendar not found.
Call stack

Function Location

1 () JROOT/libraries/src/HTML/HTMLHelper.php:149
2 Joomla\CMS\HTML\HTMLHelper::_() JROOT/plugins/content/jssearchjobs/jssearchjobs.php:28
3 require_once() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:219
4 Joomla\CMS\Application\CMSApplication->loadPluginFromFilesystem() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:160
5 Joomla\CMS\Application\CMSApplication->loadExtension() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:94
6 Joomla\CMS\Application\CMSApplication->bootPlugin() JROOT/libraries/src/Plugin/PluginHelper.php:235
7 Joomla\CMS\Plugin\PluginHelper::import() JROOT/libraries/src/Plugin/PluginHelper.php:193
8 Joomla\CMS\Plugin\PluginHelper::importPlugin() JROOT/libraries/src/MVC/Model/FormBehaviorTrait.php:176
9 Joomla\CMS\MVC\Model\ListModel->preprocessForm() JROOT/libraries/src/MVC/Model/FormBehaviorTrait.php:117
10 Joomla\CMS\MVC\Model\ListModel->loadForm() JROOT/libraries/src/MVC/Model/ListModel.php:445
11 Joomla\CMS\MVC\Model\ListModel->getFilterForm() JROOT/administrator/components/com_installer/src/View/Updatesites/HtmlView.php:79
12 Joomla\Component\Installer\Administrator\View\Updatesites\HtmlView->display() JROOT/administrator/components/com_installer/src/Controller/DisplayController.php:74
13 Joomla\Component\Installer\Administrator\Controller\DisplayController->display() JROOT/libraries/src/MVC/Controller/BaseController.php:730
14 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/libraries/src/Dispatcher/ComponentDispatcher.php:146
15 Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:389
16 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:143
17 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:186
18 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:278
19 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/includes/app.php:63
20 require_once() JROOT/administrator/index.php:32

@timpennington The error comes from a third party plugin content jssearchjobs which you are using on your site. That plugin uses a method which is removed in Joomla 4. You should disable that plugin for now and contact the developer of that plugin and ask him to fix it. It's not an issue with Joomla core .

please i have the same error here

can help me please to know where is the problem?
plugin?
css?

thank so much to everything

@simonevecchi that is a closed issue. If the explanations here don't help you to solve the problem, please open a post in the forum: forum.joomla.org