Markers not displaying where expected are almost always due to problems with the Google API key, and typically because of URL referrer restrictions added.
If you go to Toolset > Settings > Maps, where you enter the API key you'll see a button to test the key. If unsuccessful that is why your markers fail to load.
The search page is currently being edited (by you) so I can't take a proper look, but it appears to me that on that page you have a row where you insert the search form on the left and in the same row insert the map (id 6) on the right.
Then on the row below on the right you insert the View results.
The View in question (Forellensee-Suche) inserts the map (id 6) in the Loop Editor, and also inserts the marker (but not inside the wpv-loop tags where it belongs), and then in the Output Editor you have again added the same map (id 6), although the marker is wrongly included within the wpv-map-render shortcode.
So you are adding the same map 3 times on the same page (but they all have the same id, which is therefore failing).
I suggest you create a simple test page which you don't design with any page builder and simply insert the whole View.
Edit the View so that you remove the map and marker from the final output section, and you move the marker shortcode within the wpv-loop tags.
Check that test page. You should see the results of the View listed (I don't see any on your current page), as well as the map with a marker.
Once you have that working you should be able to port it to your desired page.
