500 - behavior::modal not found - When trying to use + button on front-end

Hello JEvents team,

after migrating from Joomla 3 to Joomla 4, I can no longer use the front-end to add events:

(https://ig-klettern-halle.de/kletterthalia/belegungsplan)

As soon as I klick the + button or the text link "Add an event", I get the error "500 - behavior::modal not found."

With debug enabled, I get the following:

Call stack

#	Function	Location

1	()	JROOT/libraries/src/HTML/HTMLHelper.php:139

2	Joomla\CMS\HTML\HTMLHelper::_()	JROOT/plugins/system/t3/base-bs3/html/layouts/joomla/editors/buttons.php:15

3	include()	JROOT/plugins/system/t3/includes/core/t3.php(528) : eval()'d code:128

4	Joomla\CMS\Layout\_FileLayout->render()	JROOT/libraries/src/Layout/LayoutHelper.php:76

5	Joomla\CMS\Layout\LayoutHelper::render()	JROOT/plugins/editors/jce/jce.php:299

6	plgEditorJCE->displayButtons()	JROOT/plugins/editors/jce/jce.php:196

7	plgEditorJCE->onDisplay()	JROOT/libraries/src/Editor/Editor.php:196

8	Joomla\CMS\Editor\Editor->display()	JROOT/libraries/src/Form/Field/EditorField.php:251

9	Joomla\CMS\Form\Field\EditorField->getInput()	JROOT/libraries/src/Form/FormField.php:471

10	Joomla\CMS\Form\FormField->__get()	JROOT/libraries/src/Form/Field/TextareaField.php:94

11	Joomla\CMS\Form\Field\TextareaField->__get()	JROOT/libraries/src/Form/Field/EditorField.php:138

12	Joomla\CMS\Form\Field\EditorField->__get()	JROOT/administrator/components/com_jevents/views/abstract/abstract.php:1249

13	JEventsAbstractView->setupEditForm()	JROOT/components/com_jevents/views/icalevent/view.html.php:196

14	ICalEventViewIcalevent->edit()	JROOT/administrator/components/com_jevents/views/abstract/abstract.php:93

15	JEventsAbstractView->display()	JROOT/administrator/components/com_jevents/controllers/icalevent.php:511

16	AdminIcaleventController->edit()	JROOT/components/com_jevents/controllers/icalevent.php:178

17	ICalEventController->edit()	JROOT/libraries/src/MVC/Controller/BaseController.php:672

18	Joomla\CMS\MVC\Controller\BaseController->execute()	JROOT/components/com_jevents/jevents.php:339

19	require_once()	JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:71

20	Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}()	JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:73

21	Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch()	JROOT/libraries/src/Component/ComponentHelper.php:355

22	Joomla\CMS\Component\ComponentHelper::renderComponent()	JROOT/libraries/src/Application/SiteApplication.php:200

23	Joomla\CMS\Application\SiteApplication->dispatch()	JROOT/libraries/src/Application/SiteApplication.php:241

24	Joomla\CMS\Application\SiteApplication->doExecute()	JROOT/libraries/src/Application/CMSApplication.php:294

25	Joomla\CMS\Application\CMSApplication->execute()	JROOT/includes/app.php:61

26	require_once()	JROOT/index.php:32

I already tried to disable various plugins with no success so far.
Does the debug output provide any hint which plugin or component might be the trouble maker?

Thanks for any suggestions/help,
Bjoern