Hi, I am getting the same error message. (There is an error with your API key, it has been rejected by Google API Service. => Internal Server Error)
I created a new Google API project, made the YouTube Data API v3 available and set some domain names that are allowed with this key.
I am using the plugin version 1.9 and WordPress version 4.8.1.
For “Query type” “Search for keywords” I tried entering any keyword (e.g. “Mediathek Jugendarbeit”) with no success.
For “Query type” “Channel” I tried entering the channel id you provided earlier (UCOt1-IALzefRHgpll2q97ig) and for “Playlist id” I tried PLMC9KNkIncKtPzgY-5rmhvj7fax8fdxoj with no success.
Could you please assist?
Thanks so much
Christina