Hi,
The error message
plgFinderHikashopBridge::onFinderAfterSave(): Argument #1 ($event) must be of type Joomla\CMS\Event\Finder\AfterSaveEvent, Joomla\Event\Event given, called in /var/www/vhosts/kulturforum-freiburg.info/entwicklung.kulturforum-freiburg.info/libraries/vendor/joomla/event/src/Dispatcher.php on line 454
indicates that an extension on your website is firing the event onFinderAfterSave providing it with an object of the class Joomla\Event\Event while HikaShop's finder (smart search) plugin expects it to be of class Joomla\CMS\Event\Finder\AfterSaveEvent
Simply disabling the HikaShop finder plugin via the Joomla plugins manager will remove the error.
However, the problem is not in the HikaShop finder plugin.
If you check the file plugins/finder/content/src/Extension/Content.php (which is the main file of the Joomla articles finder plugin (which indexes Joomla articles for the smart search), you can see on line 175 :
public function onFinderAfterSave(FinderEvent\AfterSaveEvent $event): void
And FinderEvent is actually an alias of Joomla\CMS\Event\Finder.
So the default content finder plugin of Joomla also expects Joomla\CMS\Event\Finder\AfterSaveEvent
Basically, that means that the problem is not on our end, but in the code which triggers the onFinderAfterSave event on your website.
We actually had a similar issue with HikaShop 5.0.0 because with Joomla 5, events now have specific event classes while before, it was the generic Joomla\Event\Event class.
So I think the issue comes from an extension which needs to be updated to support the new way of triggering these events on Joomla 5 I think.