Thanks for the quick response. beta has already been updated to 4.0.31, here's with debug output:
https://beta.ias-observatory.org/index.php/de/publikationen implode() expects at most 2 arguments, 3 given
Call stack
# Function Location
1 () JROOT/components/com_jdownloads/src/Helper/CategoriesHelper.php:397
2 implode() JROOT/components/com_jdownloads/src/Helper/CategoriesHelper.php:397
3 JDownloads\Component\JDownloads\Site\Helper\CategoriesHelper->_load() JROOT/components/com_jdownloads/src/Helper/CategoriesHelper.php:186
4 JDownloads\Component\JDownloads\Site\Helper\CategoriesHelper->get() JROOT/components/com_jdownloads/src/Model/CategoriesModel.php:221
5 JDownloads\Component\JDownloads\Site\Model\CategoriesModel->getItems() JROOT/libraries/src/MVC/View/AbstractView.php:146
6 Joomla\CMS\MVC\View\AbstractView->get() JROOT/components/com_jdownloads/src/View/Categories/HtmlView.php:81
7 JDownloads\Component\JDownloads\Site\View\Categories\HtmlView->display() JROOT/libraries/src/MVC/Controller/BaseController.php:645
8 Joomla\CMS\MVC\Controller\BaseController->display() JROOT/components/com_jdownloads/src/Controller/DisplayController.php:123
9 JDownloads\Component\JDownloads\Site\Controller\DisplayController->display() JROOT/libraries/src/MVC/Controller/BaseController.php:678
10 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/libraries/src/Dispatcher/ComponentDispatcher.php:143
11 Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() JROOT/components/com_jdownloads/src/Dispatcher/Dispatcher.php:60
12 JDownloads\Component\JDownloads\Site\Dispatcher\Dispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:361
13 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/SiteApplication.php:208
14 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/SiteApplication.php:249
15 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:293
16 Joomla\CMS\Application\CMSApplication->execute() JROOT/includes/app.php:61
17 require_once() JROOT/index.php:32