I reactivated the live page. You can see the error at
www.hobby600.de. Strangely it worked until a few days ago. It also works on my local installation.
ERROR 500 - JHtml jemhtml not found. in /is/htdocs/######/libraries/src/HTML/HTMLHelper.php:122
Call stack
# Function Location
1 () JROOT/libraries/src/HTML/HTMLHelper.php:122
2 Joomla\CMS\HTML\HTMLHelper::_() JROOT/components/com_jem/classes/output.class.php:166
3 JemOutput::submitbutton() JROOT/components/com_jem/classes/output.class.php:85
4 JemOutput::createButtonBar() JROOT/components/com_jem/views/event/tmpl/responsive/default.php:39
5 include() JROOT/libraries/src/MVC/View/HtmlView.php:701
6 Joomla\CMS\MVC\View\HtmlView->loadTemplate() JROOT/libraries/src/MVC/View/HtmlView.php:230
7 Joomla\CMS\MVC\View\HtmlView->display() JROOT/components/com_jem/views/event/view.html.php:333
8 JemViewEvent->display() JROOT/components/com_jem/controller.php:104
9 JemController->display() JROOT/libraries/src/MVC/Controller/BaseController.php:702
10 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/components/com_jem/jem.php:45
11 require_once() JROOT/libraries/src/Component/ComponentHelper.php:402
12 Joomla\CMS\Component\ComponentHelper::executeComponent() JROOT/libraries/src/Component/ComponentHelper.php:377
13 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/SiteApplication.php:194
14 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/SiteApplication.php:233
15 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:225
16 Joomla\CMS\Application\CMSApplication->execute() JROOT/index.php:49