No youtube playlist videos to be found - maybe API key not set ? This is the feed - https://www.googleapis.com/youtube/v3/playlistItems?part=snippet&playlistId=PLD1UgPQmL1N5cT7W1j0c-sfSFMij_ZWwm&key=AIzaSyCxz4pAMRB4zFqCRXG1jtxe-vB0oH_cg30&maxResults=50
There is a per-IP or per-Referer restriction configured on your API key and the request does not match these restrictions. Please use the Google Developers Console to update your API key configuration if request from this IP or referer should be allowed.
Suggestion - go to Video Gallery > Settings and enter your YouTube API Key

Levenslessen die wellicht vooruit kunnen helpen.