I have the same problem, on a clean install of J4.0.5 and Community Builder.
0 Call to a member function get() on string
Call stack : (for when I try to create a new Topic)
# Function Location
1 () JROOT/plugins/kunena/comprofiler/:75
2 KunenaActivityComprofiler->onBeforePost() JROOT/libraries/kunena/src/Integration/:97
3 Kunena\Forum\Libraries\Integration\KunenaActivity->__call() JROOT/components/com_kunena/src/Controllers/:791
4 Kunena\Forum\Site\Controllers\TopicController->post() JROOT/libraries/kunena/src/Controller/:349
5 Kunena\Forum\Libraries\Controller\KunenaController->executeTask() JROOT/libraries/kunena/src/Controller/:240
6 Kunena\Forum\Libraries\Controller\KunenaController->execute() JROOT/components/com_kunena/src/Dispatcher/:113
7 Kunena\Forum\Site\Dispatcher\Dispatcher->dispatch() JROOT/libraries/src/Component/:389
8 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/:206
9 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/:245
10 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/:278
11 Joomla\CMS\Application\CMSApplication->execute() JROOT/includes/:63
12 require_once() JROOT/:32
and
404 Affichage introuvable [name, type, prefix] : topic, html, site
Call stack : when I try to open the 'sample data' post that is created when I pressed the install Sample Data
# Function Location
1 () JROOT/libraries/src/MVC/Controller/:898
2 Joomla\CMS\MVC\Controller\BaseController->getView() JROOT/libraries/kunena/src/Controller/:430
3 Kunena\Forum\Libraries\Controller\KunenaController->display() JROOT/libraries/kunena/src/Controller/:349
4 Kunena\Forum\Libraries\Controller\KunenaController->executeTask() JROOT/libraries/kunena/src/Controller/:240
5 Kunena\Forum\Libraries\Controller\KunenaController->execute() JROOT/components/com_kunena/src/Dispatcher/:113
6 Kunena\Forum\Site\Dispatcher\Dispatcher->dispatch() JROOT/libraries/src/Component/:389
7 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/:206
8 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/:245
9 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/:278
10 Joomla\CMS\Application\CMSApplication->execute() JROOT/includes/:63
11 require_once() JROOT/:32
PHP 8.0.14
MySQLi 10.5.12-MariaDB-0+deb11u1
