When I log into the front end of my Purity III site and click on an "Edit Published Page" link, I get a "500 - behavior::tabstate not found." error. I suspected the template so I switched to the Cassiopeia template and the error went away. Any clues?

I've made sure that J4, T3 Framework and Purity III are all up to date.

With debug switched on, the error report looks like this:

Call stack

Function Location

1 () JROOT/libraries/src/HTML/HTMLHelper.php:139
2 Joomla\CMS\HTML\HTMLHelper::_() JROOT/plugins/system/t3/base-bs3/html/com_content/form/edit.php:12
3 include() JROOT/plugins/system/t3/includes/core/t3.php(528) : eval()'d code:410
4 Joomla\CMS\MVC\View_JHtmlView->loadTemplate() JROOT/plugins/system/t3/includes/core/t3.php(528) : eval()'d code:203
5 Joomla\CMS\MVC\View_JHtmlView->display() JROOT/components/com_content/src/View/Form/HtmlView.php:197
6 Joomla\Component\Content\Site\View\Form\HtmlView->display() JROOT/libraries/src/MVC/Controller/BaseController.php:639
7 Joomla\CMS\MVC\Controller\BaseController->display() JROOT/components/com_content/src/Controller/DisplayController.php:121
8 Joomla\Component\Content\Site\Controller\DisplayController->display() JROOT/libraries/src/MVC/Controller/BaseController.php:672
9 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/libraries/src/Dispatcher/ComponentDispatcher.php:143
10 Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() JROOT/components/com_content/src/Dispatcher/Dispatcher.php:56
11 Joomla\Component\Content\Site\Dispatcher\Dispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:355
12 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/SiteApplication.php:200
13 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/SiteApplication.php:241
14 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:294
15 Joomla\CMS\Application\CMSApplication->execute() JROOT/includes/app.php:61
16 require_once() JROOT/index.php:32