Google API key setting is not recognized

Hello,

I am using the Events Manager for a few weeks now.
Everything works fine, except the maps that are shown on the single event pages.
In the beginning this worked fine, but it stopped.

I think this is connected to the fact, that I created a single-event.php file.
Somehow my Google API key is not used anymore. So i get this error message (console):
Google Maps API error: MissingKeyMapError

I have added an API key in the settings and maps are working fine on the page, where you can create new events. Here the API key is used correctly.

Do I have to define something special in my single-event.php file to make sure, that my API key is used again?