I’ve tried all Combinations:

• filled in the API, clicked on “API-KEY überprüfen” (quitted by the message “Mit diesem API-Schlüssel konnte keine Verbindung zu Google Maps hergestellt werden.” an despite of this message also on “Alle Änderungen speichern”
• filled in the API, clicked on “Alle Änderungen speichern” and then on “API-KEY überprüfen” (also getting the same erro message)

I also recognized, that the API sometimes is lost, when i open WordPress again. But this effect seems not to be permanent. Sometimes it is saved, sometimes it seems to be lost..